Segment and Geographic Information - Segment Reporting Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2013	Jun. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Brokerage revenues:
Rates	$ 109,110		$ 131,359	$ 392,401	$ 412,645
Credit	54,410		67,926	190,895	222,381
Foreign Exchange	47,393		48,910	167,433	160,881
Equities and Other Asset Classes	34,862		34,545	115,524	120,082
Real Estate	105,303		96,551	281,707	226,518
Real estate management services	40,447		39,672	119,608	81,563
Fees from related parties	8,071		13,102	33,461	39,143
Market data	1,178		4,166	8,946	13,120
Software solutions	444		2,485	5,540	7,421
Other revenues	33,269		3,199	35,274	4,030
Gain on divestiture		723,147		723,147
Losses on equity investments	(2,705)		(2,995)	(7,217)	(8,103)
Total non-interest revenues	431,782		438,920	2,066,719	1,279,681
Interest income	1,563		1,397	4,762	5,135
Total revenues	433,345		440,317	2,071,481	1,284,816
Interest expense	9,164		9,758	28,853	24,894
Non-interest expenses	381,518		426,947	1,778,017	1,232,948
Income from operations before income taxes	42,663		3,612	264,611	26,974
Corporate Items [Member]
Brokerage revenues:
Rates
Credit
Foreign Exchange
Equities and Other Asset Classes
Real Estate
Real estate management services
Fees from related parties	8,058		10,328	27,768	30,389
Market data
Software solutions
Other revenues	334		2,907	1,684	3,227
Gain on divestiture				723,147
Losses on equity investments	(2,705)		(2,995)	(7,217)	(8,103)
Total non-interest revenues	5,687		10,240	745,382	25,513
Interest income	1,214		1,193	3,722	3,926
Total revenues	6,901		11,433	749,104	29,439
Interest expense	8,754		8,104	25,958	19,902
Non-interest expenses	22,667		53,773	652,278	175,443
Income from operations before income taxes	(24,520)		(50,444)	70,868	(165,906)
Financial Services [Member] | Operating Segments [Member]
Brokerage revenues:
Rates	109,110		131,359	392,401	412,645
Credit	54,410		67,926	190,895	222,381
Foreign Exchange	47,393		48,910	167,433	160,881
Equities and Other Asset Classes	34,862		34,545	115,524	120,082
Real Estate
Real estate management services
Fees from related parties	13		2,774	5,693	8,754
Market data	1,178		4,166	8,946	13,120
Software solutions	444		2,485	5,540	7,421
Other revenues	32,935		237	33,558	283
Gain on divestiture
Losses on equity investments
Total non-interest revenues	280,345		292,402	919,990	945,567
Interest income	262		190	800	904
Total revenues	280,607		292,592	920,790	946,471
Interest expense	410		1,650	2,894	4,730
Non-interest expenses	226,613		246,700	743,895	762,572
Income from operations before income taxes	53,584		44,242	174,001	179,169
Real Estate Services [Member] | Operating Segments [Member]
Brokerage revenues:
Rates
Credit
Foreign Exchange
Equities and Other Asset Classes
Real Estate	105,303		96,551	281,707	226,518
Real estate management services	40,447		39,672	119,608	81,563
Fees from related parties
Market data
Software solutions
Other revenues			55	32	520
Gain on divestiture
Losses on equity investments
Total non-interest revenues	145,750		136,278	401,347	308,601
Interest income	87		14	240	305
Total revenues	145,837		136,292	401,587	308,906
Interest expense			4	1	262
Non-interest expenses	132,238		126,474	381,844	294,933
Income from operations before income taxes	$ 13,599		$ 9,814	$ 19,742	$ 13,711