– Controversial conservative billionaire David Koch is leaving the board of the Americans for Prosperity Foundation, the political advocacy group he founded with his brother, Charles, which made the brothers "two of the most powerful men in American politics," as USA Today puts it. David Koch, 78, is also stepping down from his roles at the brothers' Koch Industries industrial conglomerate, where he served as executive vice president and board member as well as chairman and CEO of one of the company's subsidiaries. Koch officials said deteriorating health is behind his retirement, but gave no further details. Charles Koch, 82, will remain chairman and chief executive of Koch Industries, reports the Washington Post, which notes that the aging brothers have "groom[ed] a new generation of leaders" for their massive political network.