Revenue from Contracts with Customers - Schedule of Unaudited Condensed Consolidated Statements of Financial Condition Impact of ASC Topic 606 (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets
Accrued commissions and other receivables, net	$ 885,597		$ 620,039
Total assets	8,732,296		5,429,712
Liabilities and equity
Accrued compensation	545,004		432,733
Accounts payable, accrued and other liabilities	1,067,516		942,917
Retained earnings	(798,717)		(859,009)
Total liabilities, redeemable partnership interest, and equity	8,732,296		$ 5,429,712
Revenue [Member]
Assets
Total assets		$ 64,400
Liabilities and equity
Retained earnings		$ 20,200
Revenue [Member] | ASC Topic 606 Impact [Member]
Assets
Accrued commissions and other receivables, net	108,666
Total assets	108,666
Liabilities and equity
Accrued compensation	48,057
Accounts payable, accrued and other liabilities	25,399
Retained earnings	35,210
Total liabilities, redeemable partnership interest, and equity	$ 108,666