Robert Mercer, a billionaire investor who is a big backer of many conservative causes and a patron of former White House adviser Steve Bannon, is stepping down as co-chief executive of Renaissance Technologies, the giant hedge fund…

In the letter, Mr. Mercer said he was stepping down from his post as of Jan. 1, 2018, and would also step down from the hedge fund’s board. He said he would remain active on the research side of the hedge-fund firm, which makes trades using complex mathematical equations.

The letter did not give a reason for Mr. Mercer’s decision. Mr. Mercer’s involvement in conservative politics became controversial during the presidential election.