Segment and Geographic Information - Segment Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Brokerage revenues:
Rates	$ 133,469	$ 120,678	$ 269,221	$ 240,297
Credit	70,730	77,330	152,600	162,245
Foreign exchange	79,681	76,835	159,707	159,303
Energy and commodities	48,479	57,306	101,624	121,704
Equities, insurance, and other asset classes	85,324	45,593	161,030	93,959
Leasing and other services	144,681	124,555	272,255	230,182
Real estate capital markets	98,029	82,739	176,825	144,872
Real estate management services	51,589	45,529	102,219	91,587
Fees from related parties	5,576	4,865	12,141	11,935
Data, software and post-trade	13,322	14,160	26,409	28,094
Other revenues	876	402	1,852	4,084
Total non-interest revenues	731,756	649,992	1,435,883	1,288,262
Interest income	6,001	3,778	9,304	6,162
Total revenues	737,757	653,770	1,445,187	1,294,424
Interest expense	16,676	14,624	31,497	28,082
Non-interest expenses	682,038	620,817	1,351,779	1,226,594
Total expenses	698,714	635,441	1,383,276	1,254,676
Other income (losses), net:
Gain (loss) on divestiture and sale of investments			557
Gains (losses) on equity investments	1,602	863	1,839	1,751
Other income (losses)	4,855	10,012	9,944	7,095
Total other income (losses), net	6,457	10,875	12,340	8,846
Income (loss) from operations before income taxes	45,500	29,204	74,251	48,594
Corporate Items [Member]
Brokerage revenues:
Fees from related parties	5,576	4,865	12,141	11,935
Other revenues	93	129	123	415
Total non-interest revenues	5,669	4,994	12,264	12,350
Interest income	4,454	2,274	6,154	3,485
Total revenues	10,123	7,268	18,418	15,835
Interest expense	16,676	14,624	31,497	28,082
Non-interest expenses	68,947	51,027	148,199	113,116
Total expenses	85,623	65,651	179,696	141,198
Other income (losses), net:
Gain (loss) on divestiture and sale of investments			557
Gains (losses) on equity investments	1,602	863	1,839	1,751
Other income (losses)	786	11,338	1,227	(2,551)
Total other income (losses), net	2,388	12,201	3,623	(800)
Income (loss) from operations before income taxes	(73,112)	(46,182)	(157,655)	(126,163)
Financial Services [Member] | Operating Segments [Member]
Brokerage revenues:
Rates	133,469	120,678	269,221	240,297
Credit	70,730	77,330	152,600	162,245
Foreign exchange	79,681	76,835	159,707	159,303
Energy and commodities	48,479	57,306	101,624	121,704
Equities, insurance, and other asset classes	85,324	45,593	161,030	93,959
Data, software and post-trade	13,322	14,160	26,409	28,094
Other revenues	655	197	1,597	3,593
Total non-interest revenues	431,660	392,099	872,188	809,195
Interest income	657	641	1,307	1,152
Total revenues	432,317	392,740	873,495	810,347
Non-interest expenses	351,579	340,758	705,485	685,872
Total expenses	351,579	340,758	705,485	685,872
Other income (losses), net:
Other income (losses)	4,069	(1,326)	8,717	9,646
Total other income (losses), net	4,069	(1,326)	8,717	9,646
Income (loss) from operations before income taxes	84,807	50,656	176,727	134,121
Real Estate Services [Member] | Operating Segments [Member]
Brokerage revenues:
Leasing and other services	144,681	124,555	272,255	230,182
Real estate capital markets	98,029	82,739	176,825	144,872
Real estate management services	51,589	45,529	102,219	91,587
Other revenues	128	76	132	76
Total non-interest revenues	294,427	252,899	551,431	466,717
Interest income	890	863	1,843	1,525
Total revenues	295,317	253,762	553,274	468,242
Non-interest expenses	261,512	229,032	498,095	427,606
Total expenses	261,512	229,032	498,095	427,606
Other income (losses), net:
Income (loss) from operations before income taxes	$ 33,805	$ 24,730	$ 55,179	$ 40,636