Segment and Geographic Information - Segment Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Brokerage revenues:
Rates	$ 123,041	$ 112,384	$ 392,262	$ 352,681
Credit	66,133	67,221	218,733	229,466
Foreign exchange	83,899	73,191	243,606	232,494
Energy and commodities	48,231	47,061	149,855	168,765
Equities, insurance, and other asset classes	79,657	39,076	240,687	133,035
Leasing and other services	158,604	139,109	430,859	369,291
Real estate capital markets	98,307	91,098	270,865	234,780
Gains from mortgage banking activities, net	45,455	65,377	164,263	139,009
Real estate management and other services	60,798	49,373	163,017	140,960
Servicing fees	29,057	22,502	80,729	62,199
Fees from related parties	7,173	6,354	20,129	19,086
Data, software and post-trade	13,776	13,266	40,185	41,360
Other revenues	1,171	796	3,023	4,880
Total non-interest revenues	815,302	726,808	2,418,213	2,128,006
Interest income	11,726	8,012	40,909	24,332
Total revenues	827,028	734,820	2,459,122	2,152,338
Interest expense	24,425	18,831	69,678	53,770
Non-interest expenses	750,547	670,908	2,197,440	1,973,158
Total expenses	774,972	689,739	2,267,118	2,026,928
Other income (losses), net:
Gain (loss) on divestiture and sale of investments	4	7,044	561	7,044
Gains (losses) on equity investments	2,147	796	3,986	2,547
Other income (losses)	88,195	91,655	97,928	98,192
Total other income (losses), net	90,346	99,495	102,475	107,783
Income (loss) from operations before income taxes	142,402	144,576	294,479	233,193
Corporate Items [Member]
Brokerage revenues:
Fees from related parties	6,933	6,126	19,074	18,061
Other revenues	35	138	158	553
Total non-interest revenues	6,968	6,264	19,232	18,614
Interest income	3,987	1,118	10,359	4,676
Total revenues	10,955	7,382	29,591	23,290
Interest expense	19,988	16,601	53,913	44,833
Non-interest expenses	89,178	85,539	237,532	198,424
Total expenses	109,166	102,140	291,445	243,257
Other income (losses), net:
Gain (loss) on divestiture and sale of investments	4	7,044	561	7,044
Gains (losses) on equity investments	2,147	796	3,986	2,547
Other income (losses)	(901)	21,762	115	18,653
Total other income (losses), net	1,250	29,602	4,662	28,244
Income (loss) from operations before income taxes	(96,961)	(65,156)	(257,192)	(191,723)
Financial Services [Member] | Operating Segments [Member]
Brokerage revenues:
Rates	123,041	112,384	392,262	352,681
Credit	66,133	67,221	218,733	229,466
Foreign exchange	83,899	73,191	243,606	232,494
Energy and commodities	48,231	47,061	149,855	168,765
Equities, insurance, and other asset classes	79,657	39,076	240,687	133,035
Data, software and post-trade	13,776	13,266	40,185	41,360
Other revenues	1,096	645	2,693	4,238
Total non-interest revenues	415,833	352,844	1,288,021	1,162,039
Interest income	824	743	2,131	1,895
Total revenues	416,657	353,587	1,290,152	1,163,934
Non-interest expenses	338,440	294,763	1,043,925	980,635
Total expenses	338,440	294,763	1,043,925	980,635
Other income (losses), net:
Other income (losses)	12,128	69,893	20,845	79,539
Total other income (losses), net	12,128	69,893	20,845	79,539
Income (loss) from operations before income taxes	90,345	128,717	267,072	262,838
Real Estate Services [Member] | Operating Segments [Member]
Brokerage revenues:
Leasing and other services	158,604	139,109	430,859	369,291
Real estate capital markets	98,307	91,098	270,865	234,780
Gains from mortgage banking activities, net	45,455	65,377	164,263	139,009
Real estate management and other services	60,798	49,373	163,017	140,960
Servicing fees	29,057	22,502	80,729	62,199
Fees from related parties	240	228	1,055	1,025
Other revenues	40	13	172	89
Total non-interest revenues	392,501	367,700	1,110,960	947,353
Interest income	6,915	6,151	28,419	17,761
Total revenues	399,416	373,851	1,139,379	965,114
Interest expense	4,437	2,230	15,765	8,937
Non-interest expenses	322,929	290,606	915,983	794,099
Total expenses	327,366	292,836	931,748	803,036
Other income (losses), net:
Other income (losses)	76,968		76,968
Total other income (losses), net	76,968		76,968
Income (loss) from operations before income taxes	$ 149,018	$ 81,015	$ 284,599	$ 162,078