Paul Allen hasn't been involved in the day-to-day operations of Microsoft (NASDAQ:MSFT) in more than three decades, yet he largely owes his vast fortune to the company he co-founded with Bill Gates in 1975.

The self-described "idea man," Allen came up with (among other things) Microsoft's name, and at one point, owned 36% of the company. But over the years, Allen's stake has eroded, and he's channeled his vast fortune into a variety of different endeavors. Let's take a look at Allen's current holdings.

Sports, art, and real estate
Allen is a member of several exclusive groups. In addition to being one of the world's richest people, he's also among the few that own multiple major sports franchises. Allen's holdings include the NFL's Seattle Seahawks, the NBA's Portland Trail Blazers, and a stake in Major League Soccer's Seattle Sounders FC.

Paul Allen. Source: by Michael Sprague via Wikimedia Commons.

The Seahawks are worth an estimated $1.3 billion, according to Forbes, while the Trail Blazers are valued at just under $1 billion. Both represent solid returns -- Allen acquired the teams for a combined value of less than $300 million.

He also dabbles in art. Allen has an extensive collection, including pieces from such renowned artists as Monet, Manet, and Georgia O'Keeffe. In recent years, he's been unloading some of his abstract pieces. In 2013, he sold a Barnett Newman painting for nearly $44 million, and last year, he parted with a Rothko for $56.2 million.

Like many investors, Allen has a significant real estate portfolio, but over the years, some of his holdings have been a bit unusual. Allen once owned a private island off the coast of Washington state, for example, though he sold it undeveloped in 2013 for $8 million. He owns numerous mansions, including his own 10,000 square-foot home, ranches, apartments, and even a movie theater (one of the most advanced in the world).

He has an extensive collection of vehicles, including two of the world's largest yachts. Allen's 414-foot Octopus is one of the longest in the world. Tatoosh is a bit smaller (303 feet) but still ranks among the top 50. Allen also collects World War II-era airplanes -- he owns several dozen.

Allen has had his fair share of bad bets
But despite his sports-related wins and lavish holdings, Allen has taken some significant lumps over the years. In 1999, he was the world's third-richest person, with an estimated net worth of $30 billion. But today, his holdings are valued at significantly less. What went wrong?

For decades, Allen made investments based on a concept he called the "Wired World" -- a notion that society was marching toward an era of connectivity, one built around the intersection of the Internet, mass media, and PCs.

It's hard to say that Allen was wrong per se -- the Internet is all-encompassing; media reaches people through smart devices on a continuous basis -- but many of his investments just didn't work out. From 1999 to 2004, he lost roughly $12 billion as the value of the dot-com companies he invested in plummeted.

His most significant blow, however, may have come in 2009, when Charter Communications -- one of the largest cable companies in the U.S. -- filed for bankruptcy. At the time, Allen was the largest shareholder, and he was wiped out almost entirely, losing a stake that had once been valued at around $7 billion in the process.

No. 51
But not all of Allen's investments went bad. Vulcan Capital, his private investment company, has positions in a variety of successful tech companies, including Alibaba, Uber, Palantir, and flipkart. And he still owns an estimated 100 million shares of Microsoft: around 1% of the company.

With a total fortune worth $17.5 billion, according to Forbes, Allen is the world's 51st richest person.