Segment Information - Segment's Contribution with Respect to Net Revenue, Operating Expenses, Operating Income (Loss) and Total Assets (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Net Revenue (Expense)	$ 633,940	$ 566,211	$ 566,896	$ 533,400	$ 612,690	$ 480,354	$ 490,405	$ 401,903	$ 2,300,447	$ 1,985,352	$ 1,912,448
Operating expenses	434,347	452,499	467,916	426,220	539,536	399,236	449,467	380,306	1,780,982	1,768,545	1,788,563
Operating Income (Loss)	199,593	113,712	98,980	107,180	73,154	81,118	40,938	21,597	519,465	216,807	123,885
Total Assets	3,332,236				3,011,137				3,332,236	3,011,137	2,986,893
Operating Segments [Member] | Financial Advisory [Member]
Segment Reporting Information [Line Items]
Net Revenue (Expense)									1,206,734	980,577	1,049,090
Operating expenses									977,681	959,668	1,057,620
Operating Income (Loss)									229,053	20,909	(8,530)
Total Assets	785,557				714,708				785,557	714,708	793,007
Operating Segments [Member] | Asset Management [Member]
Segment Reporting Information [Line Items]
Net Revenue (Expense)									1,134,595	1,039,130	896,260
Operating expenses									749,345	704,045	659,502
Operating Income (Loss)									385,250	335,085	236,758
Total Assets	588,403				612,018				588,403	612,018	566,677
Corporate [Member]
Segment Reporting Information [Line Items]
Net Revenue (Expense)									(40,882)	(34,355)	(32,902)
Operating expenses									53,956	104,832	71,441
Operating Income (Loss)									(94,838)	(139,187)	(104,343)
Total Assets	$ 1,958,276				$ 1,684,411				$ 1,958,276	$ 1,684,411	$ 1,627,209