Reports have emerged that billionaire David Koch has decided to retire from his family’s conglomerate, Koch Industries. He has served as executive vice president of the company and a member of its board. He also held the titles of chairman and CEO of one its subsidiaries, Koch Chemical Technology Group.

He also is stepping aside as chairman of the Americans for Prosperity Foundation. Americans for Prosperity has operations in 36 states and a standing army of some three million activists. It is one of the most active and well-funded political forces in the country and has been instrumental in setting the Republican agenda.

His brother, Charles Koch, announced the decisions in a letter to employees of Koch Industries. The letter cited David’s declining health has the reason for the decision. David is a cancer survivor, diagnosed with prostate cancer 25 years ago. Charles said David will now get the title of “director emeritus.” Charles will stay on as chairman and chief executive of Koch Industries.

Koch Industries, a Kansas-based industrial conglomerate, is the second-largest privately-held company in the United States after Cargill. The firm has an annual revenue of more than $100 billion. David, 78, and Charles, 82, are tied as the country’s eighth richest men. According to Forbes Magazine, both are worth about $60 billion.

The two billionaire brothers also helm a powerful conservative political network. The Koch network, whose formal name is the Seminar Network, now has 700 members who each commit to contributing a minimum of $100,000 annually to Koch-affiliated groups. The network plans to spend between $300 million and $400 million on politics and policy during the 2018 election cycle. That’s up from $250 million during the 2016 elections.

David Koch, who lives in New York with his wife of 20 years, was raised in Wichita. According to an official bio, he has pledged or contributed more than $1.3 billion to cancer research, medical centers, arts and cultural institutions, education institutions, and public policy organizations.