Segment and Geographic Information - Segment Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Brokerage revenues:
Rates	$ 118,517	$ 122,011
Credit	84,527	67,175
Foreign exchange	78,020	72,941
Energy and commodities	67,497	29,404
Equities and other asset classes	51,205	36,215
Leasing and other services	105,627	103,563
Real estate capital markets	62,133	53,742
Real estate management services	46,058	40,602
Fees from related parties	7,070	6,606
Data, software and post-trade	12,317	11,527
Other revenues	3,682	2,076
Total non-interest revenues	636,653	545,862
Interest income	2,383	1,705
Total revenues	639,036	547,567
Interest expense	13,458	15,902
Non-interest expenses	602,088	527,183
Total expenses	615,546	543,085
Other income (losses), net:
Gain on divestiture and sale of investments		(215)
Gain (losses) on equity method investments	558	803
Other income (losses)	(2,917)	31,200
Total other income (losses), net	(2,359)	31,788
Income (loss) from operations before income taxes	21,131	36,270
Corporate Items [Member]
Brokerage revenues:
Fees from related parties	7,070	6,581
Other revenues	286	223
Total non-interest revenues	7,356	6,804
Interest income	1,211	1,021
Total revenues	8,567	7,825
Interest expense	13,458	15,430
Non-interest expenses	70,603	68,048
Total expenses	84,061	83,478
Other income (losses), net:
Gain (losses) on equity method investments	558	803
Other income (losses)	(13,889)	28,267
Total other income (losses), net	(13,331)	29,070
Income (loss) from operations before income taxes	(88,825)	(46,583)
Financial Services [Member] | Operating Segments [Member]
Brokerage revenues:
Rates	118,517	122,011
Credit	84,527	67,175
Foreign exchange	78,020	72,941
Energy and commodities	67,497	29,404
Equities and other asset classes	51,205	36,215
Fees from related parties		25
Data, software and post-trade	12,317	11,527
Other revenues	3,396	1,659
Total non-interest revenues	415,479	340,957
Interest income	510	396
Total revenues	415,989	341,353
Interest expense		472
Non-interest expenses	333,831	276,982
Total expenses	333,831	277,454
Other income (losses), net:
Gain on divestiture and sale of investments		(215)
Other income (losses)	10,972	2,933
Total other income (losses), net	10,972	2,718
Income (loss) from operations before income taxes	93,130	66,617
Real Estate Services [Member] | Operating Segments [Member]
Brokerage revenues:
Leasing and other services	105,627	103,563
Real estate capital markets	62,133	53,742
Real estate management services	46,058	40,602
Other revenues		194
Total non-interest revenues	213,818	198,101
Interest income	662	288
Total revenues	214,480	198,389
Non-interest expenses	197,654	182,153
Total expenses	197,654	182,153
Other income (losses), net:
Income (loss) from operations before income taxes	$ 16,826	$ 16,236