Segment Operating Results (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
Net Revenues	[1]	$ 308,951	$ 227,161	$ 815,030	$ 593,970
Operating Expenses		238,840	178,905	644,250	474,350
Other Expenses	[2]	58,213	8,910	116,773	16,525
Operating Income (Loss)		11,898	39,346	54,007	103,095
Income (Loss) from Equity Method Investments		929	1,102	4,034	3,381
Pre-Tax Income (Loss)		12,827	40,448	58,041	106,476
Identifiable Segment Assets		1,330,389	1,223,751	1,330,389	1,223,751	$ 1,446,556
Investment Banking [Member]
Segment Reporting Information [Line Items]
Net Revenues	[1]	285,918	203,028	746,875	522,202
Operating Expenses		220,565	156,549	586,200	410,832
Other Expenses	[2]	30,131	8,828	85,168	16,279
Operating Income (Loss)		35,222	37,651	75,507	95,091
Income (Loss) from Equity Method Investments		(528)	(48)	238	455
Pre-Tax Income (Loss)		34,694	37,603	75,745	95,546
Identifiable Segment Assets		919,000	719,351	919,000	719,351
Investment Management [Member]
Segment Reporting Information [Line Items]
Net Revenues	[1]	23,033	24,133	68,155	71,768
Operating Expenses		18,275	22,356	58,050	63,518
Other Expenses	[2]	28,082	82	31,605	246
Operating Income (Loss)		(23,324)	1,695	(21,500)	8,004
Income (Loss) from Equity Method Investments		1,457	1,150	3,796	2,926
Pre-Tax Income (Loss)		(21,867)	2,845	(17,704)	10,930
Identifiable Segment Assets		$ 411,389	$ 504,400	$ 411,389	$ 504,400

[1] Net revenues include Other Revenue, net, allocated to the segments as follows: For the Three Months Ended September 30, For the Nine Months Ended September 30, 2015 2014 2015 2014Investment Banking (A)$357 $850 $(2,874) $(731)Investment Management (B)(779) (644) (2,243) (1,725)Total Other Revenue, net$(422) $206 $(5,117) $(2,456)(A)Investment Banking Other Revenue, net, includes interest expense on the Senior Notes, subordinated borrowings and line of credit of $1,452 and $4,889 for the three and nine months ended September 30, 2015, respectively, and $1,134 and $3,344 for the three and nine months ended September 30, 2014, respectively. (B)Investment Management Other Revenue, net, includes interest expense on the Senior Notes and line of credit of $962 and $2,874 for the three and nine months ended September 30, 2015, respectively, and $956 and $2,820 for the three and nine months ended September 30, 2014, respectively.

[2] Other Expenses are as follows: For the Three Months Ended September 30, For the Nine Months Ended September 30, 2015 2014 2015 2014Investment Banking Amortization of LP Units / Interests and Certain Other Awards$21,980 $— $66,123 $—Other Acquisition Related Compensation Charges— 592 1,537 6,371Special Charges— 3,732 2,151 3,732Professional Fees— — — 1,672Acquisition and Transition Costs538 4,122 1,928 4,122Fair Value of Contingent Consideration2,797 — 2,797 —Intangible Asset and Other Amortization4,816 382 10,632 382Total Investment Banking30,131 8,828 85,168 16,279Investment Management Special Charges28,000 — 31,348 —Acquisition and Transition Costs— — 11 —Intangible Asset and Other Amortization82 82 246 246Total Investment Management28,082 82 31,605 246Total Other Expenses$58,213 $8,910 $116,773 $16,525