Condensed Consolidated Statements of Financial Condition (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Cash and securities segregated, at cost	$ 1,024,491	$ 816,350
Limited partners, units issued	269,839,937	268,659,333
Limited partners, units outstanding	269,839,937	268,659,333