Billionaire hedge-fund manager Leon Cooperman is returning outside investor capital at year-end.

Omega Advisors is converting to a family office at the end of 2018, according to an investor letter obtained by CNBC.

"I turned 75 last April. It is my understanding that if you make it past 65 and cancer doesn’t get you, you can expect to live on average to 85. Hopefully, I can improve on that average, but in any event I don’t want to spend the rest of my life chasing the S&P 500 and focused on generating returns on investor capital," Cooperman said in the note to clients Monday.

The investor also revealed the Omega Credit Opportunities Fund managed by Sam Martini and Eric Schneider will continue under a new name and portfolio manager Rebecca Pacholder will launch a new high-yield debt and distressed securities fund. Cooperman said his family office will be a "substantial" investor in both of these funds.

Cooperman's main fund generated annualized returns of 12.4 percent since inception versus the S&P 500's 9.5 percent return including reinvested dividends, according to Institutional Investor.

Omega agreed last year to a $4.9 million settlement with the Securities and Exchange Commission after allegations of insider trading. Omega Advisors admitted to no wrongdoing. Investors redeemed $4 billion in capital during the SEC investigation, according to Institutional Investor.

Cooperman, chairman and CEO of Omega Advisors, founded the firm in 1991. It had approximately $3.6 billion in assets under management as of June 30.