Segment and Geographic Information - Segment Reporting Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Brokerage revenues:
Rates	$ 401,602,000	$ 491,740,000	$ 532,436,000
Credit	225,854,000	244,546,000	284,606,000
Foreign Exchange	215,168,000	212,120,000	208,011,000
Equities and Other Asset Classes	176,292,000	150,728,000	156,106,000
Real Estate	542,947,000	413,018,000	331,010,000
Real estate management services	163,227,000	163,353,000	122,704,000
Fees from related parties	28,379,000	41,128,000	53,159,000
Market data	6,676,000	10,137,000	17,302,000
Software solutions	2,801,000	6,201,000	9,962,000
Other revenues	17,232,000	5,177,000	4,495,000
Total non-interest revenues	1,780,178,000	1,738,148,000	1,719,791,000
Interest income	7,312,000	6,833,000	6,506,000
Total revenues	1,787,490,000	1,744,981,000	1,726,297,000
Interest expense	37,945,000	38,332,000	34,885,000
Non-interest expenses	1,796,881,000	2,193,833,000	1,676,371,000
Total expenses	1,834,826,000	2,232,165,000	1,711,256,000
Other Income (losses), net:
Gain on divestiture and sale of investments		723,147,000	52,471,000
Losses on equity method investments	(8,621,000)	(9,508,000)	(11,775,000)
Other Income	52,769,000	39,466,000	52,471,000
Total other income (losses), net	44,148,000	753,105,000	40,696,000
Income (loss) from operations before income taxes	(3,188,000)	265,921,000	55,737,000
Corporate Items [Member]
Brokerage revenues:
Real Estate	(948,000)
Fees from related parties	28,258,000	35,417,000	41,835,000
Other revenues	1,956,000	2,226,000	3,608,000
Total non-interest revenues	29,266,000	37,643,000	45,443,000
Interest income	5,088,000	5,395,000	4,829,000
Total revenues	34,354,000	43,038,000	50,272,000
Interest expense	34,712,000	34,830,000	28,382,000
Non-interest expenses	296,623,000	705,079,000	244,037,000
Total expenses	331,335,000	739,909,000	272,419,000
Other Income (losses), net:
Gain on divestiture and sale of investments		723,147,000
Losses on equity method investments	(8,621,000)	(9,508,000)	(11,775,000)
Other Income			52,471,000
Total other income (losses), net	(8,621,000)	713,639,000	40,696,000
Income (loss) from operations before income taxes	(305,602,000)	16,768,000	(181,451,000)
Financial Services [Member] | Operating Segments [Member]
Brokerage revenues:
Rates	401,602,000	491,740,000	532,436,000
Credit	225,854,000	244,546,000	284,606,000
Foreign Exchange	215,168,000	212,120,000	208,011,000
Equities and Other Asset Classes	176,292,000	150,728,000	156,106,000
Fees from related parties	121,000	5,711,000	11,324,000
Market data	6,676,000	10,137,000	17,302,000
Software solutions	2,801,000	6,201,000	9,962,000
Other revenues	14,142,000	2,517,000	367,000
Total non-interest revenues	1,042,656,000	1,123,700,000	1,220,114,000
Interest income	1,687,000	1,052,000	1,295,000
Total revenues	1,044,343,000	1,124,752,000	1,221,409,000
Interest expense	3,201,000	3,498,000	6,246,000
Non-interest expenses	860,633,000	957,138,000	1,000,865,000
Total expenses	863,834,000	960,636,000	1,007,111,000
Other Income (losses), net:
Other Income	52,769,000	39,466,000
Total other income (losses), net	52,769,000	39,466,000
Income (loss) from operations before income taxes	233,278,000	203,582,000	214,298,000
Real Estate Services [Member] | Operating Segments [Member]
Brokerage revenues:
Real Estate	543,895,000	413,018,000	331,010,000
Real estate management services	163,227,000	163,353,000	122,704,000
Other revenues	1,134,000	434,000	520,000
Total non-interest revenues	708,256,000	576,805,000	454,234,000
Interest income	537,000	386,000	382,000
Total revenues	708,793,000	577,191,000	454,616,000
Interest expense	32,000	4,000	257,000
Non-interest expenses	639,625,000	531,616,000	431,469,000
Total expenses	639,657,000	531,620,000	431,726,000
Other Income (losses), net:
Income (loss) from operations before income taxes	$ 69,136,000	$ 45,571,000	$ 22,890,000