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Bob Iger has stepped down as CEO of Disney, a move that sent immediate shock waves through the media industry that he helped shape in recent years.

Iger's move, announced Tuesday, is effective immediately. He'll stay on as executive chairman through December 2021 and continue to direct Disney's content creation — arguably the most important role at the company.

Bob Chapek, the head of Disney's lucrative parks unit, is the company's new chief executive, a job that had been sought after by at least half a dozen of Iger's current and former deputies.

Iger was named president and chief operating officer of Disney in 2000 and became CEO in 2005, taking over from Michael Eisner, whose tenure was ended by a shareholder revolt led by Roy E. Disney, nephew of the company's founder, Walt Disney, who wanted to shake up the company's management.

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Iger, who turned 69 this month, was instrumental in guiding Disney into the streaming age and through its acquisition of Fox's entertainment business — a deal worth $52.4 billion when it was announced. Under Iger, Disney went on an acquisition spree that has given it one of the deepest libraries of family-friendly content of any media company, including Pixar, Marvel and Lucasfilm.

The acquisition strategy allowed Disney to dominate the box office, generating a record $13 billion in revenue in 2019. It also allowed Disney to take on Netflix with its own streaming service, Disney+, which already has more than 30 million subscribers.

On a phone call with investors following the announcement, Iger said the change will allow him to focus on the creative side of Disney, something he couldn't do as CEO.

Iger said the decision to change titles now was "not accelerated by any particular reason."

Iger said Disney's board of directors had seen Chapek as his successor "for some time now." He said the immediate change will also give him more time to collaborate with Chapek before 2021.

Iger said two factors led to Tuesday's decision: how best to manage the company in its current form and how best to manage the succession and transition. Iger said making the change now provided the best outcome.

While the news, which was announced in a press release, came as a surprise, Iger had been planning his succession for a while. Last year, at Disney's investor conference, he said "2021 will be the time for me to finally step down."

Disney stock declined sharply after the announcement, dropping about 2.7 percent in after-hours trading.

The news is at least a short-term blow to Kevin Mayer, the head of Disney's streaming business, who was seen by many as a potential successor. However, there was speculation that Mayer could still become CEO down the line if he succeeds with Disney's direct-to-consumer businesses, which include Disney+, ESPN+ and Hulu.

This story is a developing story. Please check back for updates.