Revenues from Contracts with Customers - Schedule of Unaudited Condensed Consolidated Statement of Operations Impact of ASC Topic 606 (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue From Contracts With Customers [Line Items]
Revenues	$ 787,082		$ 1,574,248
Total Compensation and employee benefits	630,575	$ 532,590	1,230,618	$ 1,056,414
Other expenses	66,307	50,269	134,898	92,662
Total expenses	896,222	760,933	1,759,417	1,492,146
Commissions [Member]
Revenue From Contracts With Customers [Line Items]
Revenues	658,320		1,326,919
Real Estate Management And Other Services [Member]
Revenue From Contracts With Customers [Line Items]
Revenues	107,121	$ 51,589	203,999	$ 102,219
Revenue [Member] | ASC Topic 606 Impact [Member]
Revenue From Contracts With Customers [Line Items]
Revenues	35,241		64,245
Total Compensation and employee benefits	6,270		11,912
Other expenses	24,478		42,865
Total expenses	30,748		54,777
Revenue [Member] | Commissions [Member] | ASC Topic 606 Impact [Member]
Revenue From Contracts With Customers [Line Items]
Revenues	10,763		21,380
Revenue [Member] | Real Estate Management And Other Services [Member] | ASC Topic 606 Impact [Member]
Revenue From Contracts With Customers [Line Items]
Revenues	$ 24,478		$ 42,865