(Recasts to add history, comments from Bloomberg and Doctoroff)
Sept 3 (Reuters) - Former New York Mayor Michael Bloomberg will return as head of Bloomberg LP, the data and financial news company he founded in 1981, replacing Daniel Doctoroff who has decided to step aside, the company said on Wednesday.
The privately held company, in which Bloomberg is the majority shareholder, said Doctoroff would step aside as president and chief executive at year-end.
Bloomberg, whose fortune is estimated at more than $32 billion, had expected to spend much of his time on philanthropic efforts after leaving office in late 2013. Those efforts have included fights for public health and gun control.
"This is a sad day for me and my company," Bloomberg said in a statement. "I really wanted Dan to stay and continue in his leadership role. But I understand his decision."
Bloomberg, who stepped down as New York mayor last December, added he "never intended to come back to Bloomberg LP" after 12 years as New York's mayor.
Bloomberg competes with Thomson Reuters.
Doctoroff became Bloomberg LP president in 2008 and CEO in July 2011. During that period, the company's revenues increased from $5.4 billion to more than $9 billion, he said in a memo to the company's employees.
"So why have I decided to leave now?," Doctoroff said in the memo. "Simply put, while Mike never intended to come back full time, after he left City Hall and started to get to know the company again, he rediscovered what an exciting and incredible place this is. So he naturally wanted to be more involved."
Doctoroff, 56, a former New York deputy mayor, told the New York Times that he informed Bloomberg two weeks ago that he intended to resign. Bloomberg encouraged him to stay.
"This wasn't the plan," Bloomberg told the Times. "It was his idea. If it was up to me, he would have stayed."
Bloomberg started the company in 1981, using a $10 million severance package he received after he was laid off after investment bank Salomon Brothers had been acquired. He had headed equity trading at Salomon.
During Doctoroff's tenure at the company, the numbers of subscriptions for its terminals have grown from 273,000 to 321,000, the company said in a statement. The company expanded from providing financial information to markets for legal, government and alternative energy information.
Doctoroff stressed he was not leaving for another opportunity, but would focus more of his time on non-profit activities. He also accepted Bloomberg's invitation to join the board of the Bloomberg Philanthropies, the former Mayor said in the statement. (Reporting by Peter Cooney in Washington and Ronald Grover in Los Angeles; Editing by Sandra Maler and Ken Wills)