Revenues from Contracts with Customers - Schedule of Unaudited Condensed Consolidated Statements of Financial Condition Impact of ASC Topic 606 (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets
Accrued commissions and other receivables, net	$ 798,848		$ 620,039
Total assets	6,848,673		5,429,712
Liabilities, Redeemable Partnership Interest, and Equity
Accrued compensation	471,329		432,733
Accounts payable, accrued and other liabilities	1,022,681		942,917
Retained deficit	(860,107)		(859,009)
Total liabilities	5,134,203		$ 4,243,556
Revenue [Member]
Assets
Total assets		$ 64,400
Liabilities, Redeemable Partnership Interest, and Equity
Retained deficit		20,200
Total liabilities		$ 41,900
Revenue [Member] | ASC Topic 606 Impact [Member]
Assets
Accrued commissions and other receivables, net	90,797
Total assets	90,797
Liabilities, Redeemable Partnership Interest, and Equity
Accrued compensation	43,664
Accounts payable, accrued and other liabilities	18,861
Retained deficit	28,272
Total liabilities	$ 90,797