The document was dated Sept. 23, 1998, and it contained, at least in retrospect, John Elway's golden ticket to a lifetime fortune.

The deal: Denver Broncos owner Pat Bowlen would give Elway the right to buy 10 percent of the Broncos for $15 million. Bowlen would also give him the option to buy another 10 percent of the franchise by forgoing the deferred salary Bowlen owed him on the condition Elway would become a special assistant to Bowlen, which would eventually lead to a COO job. That total deferred salary? About $21 million.

But there was even more that made the deal a no-lose proposition. If Elway wasn't happy with his investment, he could sell back his interest, two to five years later, for $5 million more than his original purchase price plus 8 percent interest a year. Since the Broncos weren't making cash calls, it was essentially free money.

And then came the real kicker. If Elway agreed to the deal, he would have right of first refusal to buy any other stake in the team if the Bowlen family sold to an outsider. NFL teams don't come up for sale often.

The value of NFL franchises has skyrocketed, but even then the Broncos were worth plenty. Still, Elway had the wherewithal to easily make this deal. In 1997, he made the largest business deal an NFL player had ever made off the field by selling his car dealerships for what turned out to be $82.5 million, comprised mostly of AutoNation stock. Elway had nine months to think about the offer. The option to buy a piece of the Broncos expired in June 1999. In January, he led the team to its second straight title. In May, he retired. June eventually passed and Elway never executed the deal, which was not extended and never offered again.

Broncos owner Pat Bowlen and John Elway (with head coach Mike Shanahan) at Elway's retirement in 1999. Steve Peterson/Getty Images

Sixteen and a half years later, as the Broncos are back in the Super Bowl, Elway is a paid employee of the team with no stake in the franchise. The 20 percent stake he passed on, based on a Forbes 2015 valuation of the team at $1.94 billion, is now worth $388 million, which would have been a 646 percent return on the 1998 investment, adjusted for inflation, had he made it.

We know all of these details because the man who sold the majority stake in the Broncos to the Bowlen family in 1984, Edgar Kaiser, sued more than a decade ago. Kaiser maintained that the terms of the deal stated that Bowlen had to offer any piece of the team to him before he offered it to anyone else. The courts eventually ruled for Bowlen, but not before the deal offered to Elway became public.

As part of the proceedings, Elway testified In 2004 that he didn't take the offer because he "did not see a place for himself as an executive of the team."

But Elway still could have purchased the original 10 percent offered for $15 million. And it's not only that he passed on the deal, it's where he put his money instead.

Elway and his partner in his car business, Mitch Pierce, took that exact amount -- $15 million -- and invested in a company run by a man named Sean Mueller. That company turned out to be a $150 million Ponzi scheme and Elway and his partner were among the largest single investors. Elway and Pierce withdrew $6 million from their investment, but still ended up losing more than $7 million in the scheme when it was all said and done.

Elway squandered other money in more bad investments. Instances include:

In January 1998, Elway, with his coach Mike Shanahan, invested in a laundromat franchise called Laundromax. The company said it hoped to do for laundry centers what Blockbuster did for video rentals. Like Blockbuster, it failed to live up to the future.

In 1998, Elway invested in a Hispanic media company called Quepasa. He bought 133,333 shares for $500,000. By June 2000, after the stock had fallen from $27 to $1, he sold 33,000 shares at a $62,000 loss, according to SEC filings.

In August 1999, Elway invested a "seven-figure amount" in MVP.com, an online sports e-commerce site, and became its co-chairman. A little more than a year later, it went bust.

So did his investment with Bowlen and Stan Kroenke in an Arena League team. In 2009, six years after founding the Colorado Crush, the entire league folded.

Given significant health concerns around Bowlen -- his battle with Alzheimer's was disclosed in 2014 -- Elway's name has re-emerged as a natural to at least be a part owner of the team in the near future. But it's hard to imagine he'll get anything close to the deal he passed on nearly 17 years ago.