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. “This is a sad day for me and my company,” Bloomberg said in a statement. 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.