T. Boone Pickens died Wednesday in his Dallas-area home at the age of 91 of natural causes, his foundation confirmed.

The self-described “opinionated energy investor, entrepreneur, proponent of American energy resources, and philanthropist” made billions in oil and gas ― both in its extraction, and in managing a hedge fund focused on the commodity.

Pickens spent the proceeds freely on causes he deemed important. In 2010, the “Oracle of Oil” pushed heavily for rapid, nationwide adoption of wind, solar and other green energy sources as part of his 10-year “Pickens Plan.”

Once worth an estimated $3 billion, Pickens’ fortune slid to around $500 million by 2016, thanks in part to sizable charitable giving. Pickens endorsed Bill Gates’ “Giving Pledge” and committed to donate at least half of his wealth to charitable causes.

His alma mater, Oklahoma State University, received at least $100 million. Per The Associated Press, he also gave $50 million each to the University of Texas’ M.D. Anderson Cancer Center in Houston and UT Southwestern Medical Center at Dallas.

He also gave heavily to the Republican Party. Pickens supported Swift Boat Veterans for Truth during the 2004 U.S. presidential campaign, offering a $1 million bounty to anyone who could prove the political action committee had spread falsehoods about then-candidate John Kerry.

Pickens failed to make good on the offer in 2008, after a group identified 10 lies, supported by 42 pages of Kerry’s military records and other documents.

Pickens reluctantly supported Trump in his 2016 presidential campaign, having initially backed former Florida Gov. Jeb Bush. “I didn’t think [Trump] was serious,” Pickens told reporters at a conference in 2016. “Donald always overestimates how successful he is,” he added while speaking on a panel the next day.

He also voiced support for then-candidate Trump’s highly controversial call for a ban on Muslims entering the country, even as he conceded he’s only American because his ancestors were immigrants.

Pickens’ health began deteriorating that year after he suffered a series of strokes. After he sustained a head injury in a fall in 2017, Pickens retired entirely in 2018.

“Just a year ago I felt immortal, wearing my age with pride, even joking about it,” he wrote in a LinkedIn post in July 2017. “But things have changed for me since the strokes. I clearly am in the fourth quarter, and the clock is ticking and my health is in decline, much as it is with others in my stage of life.”