Cash, Cash Equivalents, and Cash Restricted for Regulatory Purposes for Periods Presented in Consolidated Statement of Financial Condition - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement Of Cash Flows [Abstract]
Cash and cash equivalents	$ 2,279,274	$ 1,142,596	$ 1,936,560
Cash segregated for regulatory purposes	172,932	131,374	132,814
Total cash, cash equivalents, and cash segregated for regulatory purposes	$ 2,452,206	$ 1,273,970	$ 2,069,374