Segment and Geographic Information - Segment Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Gains from mortgage banking activities/originations, net	$ 51,972	$ 45,455	$ 132,764	$ 164,263
Revenue from contracts with customers	796,874		2,371,122
Servicing fees	34,948	29,057	96,207	80,729
Other revenues	4,154	1,171	6,557	3,023
Total non-interest revenues	961,308	815,302	2,856,916	2,418,213
Interest income	15,946	11,726	37,060	40,909
Total revenues	977,254	827,028	2,893,976	2,459,122
Interest expense	33,472	24,425	88,051	69,678
Non-interest expenses	834,842	750,547	2,539,680	2,197,440
Total expenses	868,314	774,972	2,627,731	2,267,118
Other income (losses), net:
Gain (loss) on divestiture and sale of investments		4		561
Gains (losses) on equity investments	1,344	2,147	9,999	3,986
Other income (losses)	108,776	88,195	141,908	97,928
Total other income (losses), net	110,120	90,346	151,907	102,475
Income (loss) from operations before income taxes	219,060	142,402	418,152	294,479
Corporate Items [Member]
Segment Reporting Information [Line Items]
Other revenues	199	35	1,052	158
Total non-interest revenues	7,020	6,968	20,106	19,232
Interest income	1,936	3,987	7,797	10,359
Total revenues	8,956	10,955	27,903	29,591
Interest expense	24,235	19,988	70,687	53,913
Non-interest expenses	92,056	89,178	298,472	237,532
Total expenses	116,291	109,166	369,159	291,445
Other income (losses), net:
Gain (loss) on divestiture and sale of investments		4		561
Gains (losses) on equity investments	1,344	2,147	5,049	3,986
Other income (losses)	18,512	(901)	37,135	115
Total other income (losses), net	19,856	1,250	42,184	4,662
Income (loss) from operations before income taxes	(87,479)	(96,961)	(299,072)	(257,192)
Brokerage Revenues, Rates [Member]
Segment Reporting Information [Line Items]
Revenue from contracts with customers	128,289	123,041	434,209	392,262
Brokerage Revenues, Foreign Exchange [Member]
Segment Reporting Information [Line Items]
Revenue from contracts with customers	90,683	83,899	288,292	243,606
Brokerage Revenues, Credit [Member]
Segment Reporting Information [Line Items]
Revenue from contracts with customers	67,111	66,133	224,687	218,733
Brokerage Revenues, Energy and Commodities [Member]
Segment Reporting Information [Line Items]
Revenue from contracts with customers	57,974	48,231	174,400	149,855
Brokerage Revenues, Equities, Insurance, and Other Asset Classes [Member]
Segment Reporting Information [Line Items]
Revenue from contracts with customers	81,272	79,657	266,991	240,687
Brokerage Revenues, Leasing and Other Services [Member]
Segment Reporting Information [Line Items]
Revenue from contracts with customers	203,913	158,604	541,433	430,859
Brokerage Revenues, Real Estate Capital Markets [Member]
Segment Reporting Information [Line Items]
Revenue from contracts with customers	115,436	98,307	318,491	270,865
Real Estate Management And Other Services [Member]
Segment Reporting Information [Line Items]
Revenue from contracts with customers	101,881	60,798	305,880	163,017
Fees From Related Parties [Member]
Segment Reporting Information [Line Items]
Revenue from contracts with customers	7,128	7,173	19,989	20,129
Fees From Related Parties [Member] | Corporate Items [Member]
Segment Reporting Information [Line Items]
Revenue from contracts with customers	6,821	6,933	19,054	19,074
Data, Software And Post-Trade [Member]
Segment Reporting Information [Line Items]
Revenue from contracts with customers	16,547	13,776	47,016	40,185
Financial Services [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Other revenues	3,876	1,096	5,272	2,693
Total non-interest revenues	445,752	415,833	1,440,867	1,288,021
Interest income	934	824	2,689	2,131
Total revenues	446,686	416,657	1,443,556	1,290,152
Non-interest expenses	345,889	338,440	1,125,170	1,043,925
Total expenses	345,889	338,440	1,125,170	1,043,925
Other income (losses), net:
Other income (losses)	(3,731)	12,128	10,648	20,845
Total other income (losses), net	(3,731)	12,128	10,648	20,845
Income (loss) from operations before income taxes	97,066	90,345	329,034	267,072
Financial Services [Member] | Brokerage Revenues, Rates [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue from contracts with customers	128,289	123,041	434,209	392,262
Financial Services [Member] | Brokerage Revenues, Foreign Exchange [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue from contracts with customers	90,683	83,899	288,292	243,606
Financial Services [Member] | Brokerage Revenues, Credit [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue from contracts with customers	67,111	66,133	224,687	218,733
Financial Services [Member] | Brokerage Revenues, Energy and Commodities [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue from contracts with customers	57,974	48,231	174,400	149,855
Financial Services [Member] | Brokerage Revenues, Equities, Insurance, and Other Asset Classes [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue from contracts with customers	81,272	79,657	266,991	240,687
Financial Services [Member] | Data, Software And Post-Trade [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue from contracts with customers	16,547	13,776	47,016	40,185
Real Estate Services [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Gains from mortgage banking activities/originations, net	51,972	45,455	132,764	164,263
Servicing fees	34,948	29,057	96,207	80,729
Other revenues	79	40	233	172
Total non-interest revenues	508,536	392,501	1,395,943	1,110,960
Interest income	13,076	6,915	26,574	28,419
Total revenues	521,612	399,416	1,422,517	1,139,379
Interest expense	9,237	4,437	17,364	15,765
Non-interest expenses	396,897	322,929	1,116,038	915,983
Total expenses	406,134	327,366	1,133,402	931,748
Other income (losses), net:
Gains (losses) on equity investments			4,950
Other income (losses)	93,995	76,968	94,125	76,968
Total other income (losses), net	93,995	76,968	99,075	76,968
Income (loss) from operations before income taxes	209,473	149,018	388,190	284,599
Real Estate Services [Member] | Brokerage Revenues, Leasing and Other Services [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue from contracts with customers	203,913	158,604	541,433	430,859
Real Estate Services [Member] | Brokerage Revenues, Real Estate Capital Markets [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue from contracts with customers	115,436	98,307	318,491	270,865
Real Estate Services [Member] | Real Estate Management And Other Services [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue from contracts with customers	101,881	60,798	305,880	163,017
Real Estate Services [Member] | Fees From Related Parties [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue from contracts with customers	$ 307	$ 240	$ 935	$ 1,055