Condensed Consolidated Statements of Financial Condition (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Cash and securities segregated, at cost	$ 784,535	$ 946,093
Limited partners, units issued	265,824,057	268,893,534
Limited partners, units outstanding	265,824,057	268,893,534