Departing NBC Entertainment chairman Bob Greenblatt said goodbye Monday to the network that he led for nearly eight years.

Shortly after NBC announced Monday that veteran executives Paul Telegdy and George Cheeks would take over as heads of NBC’s entertainment division, Greenblatt sent a memo to staff thanking colleagues. “I love this network and our parent company, but since NBC is back on track and has achieved such great success I think it’s time for me to turn to a new challenge,” Greenblatt wrote.

The news that Greenblatt had decided to leave NBC, first reported by Variety Friday, surprised the industry. Under Greenblatt, the Peacock has become the most dominant broadcaster. Over four of the last five seasons, it has ranked as the top-rated primetime network in the 18-49 demo. NBC also finished the 52-week 2017-18 season as the No. 1 network in nearly every ratings category, including total viewers — the first time since 2001-02 that NBC has taken the total-viewers crown.

In his memo Friday, Greenblatt wrote about that turnaround, and his decision to join NBC in 2011 at the behest of NBCUniversal CEO Steve Burke and Comcast CEO Brian Robbins.

“I will admit that when Steve Burke recruited me I wasn’t sure a turnaround was even possible,” Greenblatt wrote, “But Steve — and Brian — were determined to rebuild NBC and they made that clear to me from day one. I had unconditional support from them, including robust investment in the network, and that support has continued to this day.”

Read Greenblatt’s full memo to staff here:

To all my colleagues at NBC —

It is with very mixed feelings that I have decided to leave NBC after nearly eight years. I love this network and our parent company, but since NBC is back on track and has achieved such great success I think it’s time for me to turn to a new challenge.

I will admit that when Steve Burke recruited me I wasn’t sure a turnaround was even possible. But Steve — and Brian — were determined to rebuild NBC and they made that clear to me from day one. I had unconditional support from them, including robust investment in the network, and that support has continued to this day. None of this could have happened without their commitment to me and this company and I will never forget it.

Nor could any of this have happened without your commitment. I’m so proud of what we achieved together, including a 5-year run (and counting) at #1 in 18-49 and virtually every other demo. We also became the most watched network in America for the 2017-18 season which honestly was never even attainable in my mind. And while people tend to give me too much of the credit for these milestones, the truth is they only happened because of the incredible management teams across the entire company.

Each of my direct reports, your senior leadership, is at the top of their game. And I’m pleased that Paul Telegdy and George Cheeks will step into my role because I think that will create a seamless transition for everyone. They are two of the most talented people I’ve ever worked with and I’m happy that Steve decided to promote them. They’re well-known to everyone in the company and to the entertainment community as passionate, gifted executives and NBC will be in great hands with them in their new role.

I want to also acknowledge and personally thank the other senior managers — Jeff Bader, Len Fogge, Chip Sullivan, Brad Melnick, Jodi DiCenzo, Tracey Pakosta, Lisa Katz, Pearlena Igbokwe, Lee Straus, Masami Yamamoto, Meredith Ahr, and Rob Hayes — who are also the best in their respective fields. It sounds like a broken record, but this is truly a dream team. I also want to acknowledge Jennifer Salke and Kathy Mandato who made indelible contributions to the rebuilding of this company over the past seven years as well.

Beyond these leaders, I’m well aware of the depth of talent that cascades down through each of their respective teams. I know that the day-to-day operation of a broadcast network in today’s competitive environment is an extraordinary challenge, but you all make it look so effortless. And I have no doubt that will continue into the future.

I want to give special thanks to Mark Lazarus for being a great friend and partner in running the network for the last few years, not to mention making this place first class in sports. Likewise, I have to acknowledge the affiliate board and all our wonderful stations who have been in lock step with us on this turnaround from day one. Without their unwavering vision, none of this would have happened.

There are also many people at other divisions of NBCU that I will thank personally, all of whom showed me and this network great support in so many ways. This really is a unified media company that works together like I’ve never seen before, which is a testament to the incredible leadership of Steve Burke.

Finally, I want to commend all of the extraordinary show creators, producers, directors, writers, crews, and actors I’ve had the joy to work with over the years at NBC. We are nothing without their great work. I can’t name them all here, but it has been very rewarding to help bring their visions to life and thereby continue the legacy of this network. NBC is one of America’s great cultural institutions, and to have been given the opportunity to lead it into a new era is a privilege for which I will always be grateful.

I had the honor of knowing and working with Brandon Tartikoff in the mid-90s after he had left NBC and became a producer. The unmitigated pride I saw on his face whenever he talked about the network has always stayed with me. I also have known Grant Tinker’s sons for a long time, and to have heard from them that Grant was very pleased with NBC’s recent resurgence meant the world to me. And thanks to Warren Littlefield and Fred Silverman for the advice and support they each gave me when I first came aboard in 2011.

Steve has asked me to consult with Paul and George as needed in the future and I’m happy to help in any way I can. And wherever my new journey takes me, I hope to find ways to do business with many of you and maintain so many of our friendships.

This has been one of the most rewarding experiences of my life.

Bob