Cash, Cash Equivalents, and Cash Restricted for Regulatory Purposes for Periods Presented in Consolidated Statement of Financial Condition - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Statement Of Cash Flows [Abstract]
Cash and cash equivalents	$ 450,343	$ 696,283	$ 658,387
Cash segregated for regulatory purposes	26,611	90,802	214
Total cash, cash equivalents, and cash segregated for regulatory purposes	$ 476,954	$ 787,085	$ 658,601	$ 986,167