Segment Operating Results (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 Revenues	$ 321,888	$ 227,161	$ 217,696	$ 149,113	$ 218,672	$ 187,328	$ 206,797	$ 152,631	$ 915,858	[1]	$ 765,428	[1]	$ 642,373	[1]
Operating Expenses									719,474		598,806		523,386
Other Expenses									25,437	[2]	36,447	[2]	53,452	[2]
Operating Income (Loss)	67,852	39,346	43,035	20,714	43,876	31,868	38,181	16,250	170,947		130,175		65,535
Income from Equity Method Investments	1,799	1,102	2,038	241	5,993	562	1,015	756	5,180		8,326		4,852
Pre-Tax Income (Loss) from Continuing Operations	69,651	40,448	45,073	20,955	49,869	32,430	39,196	17,006	176,127		138,501		70,387
Identifiable Segment Assets	1,446,556				1,180,783				1,446,556		1,180,783		1,145,218
Investment Banking [Member]
Segment Reporting Information [Line Items]
Net Revenues									819,637	[1]	670,785	[1]	565,219	[1]
Operating Expenses									632,927		516,921		444,510
Other Expenses									25,109	[2]	33,740	[2]	50,774	[2]
Operating Income (Loss)									161,601		120,124		69,935
Income from Equity Method Investments									495		2,906		2,258
Pre-Tax Income (Loss) from Continuing Operations									162,096		123,030		72,193
Identifiable Segment Assets	934,648				693,890				934,648		693,890		624,977
Investment Management [Member]
Segment Reporting Information [Line Items]
Net Revenues									96,221	[1]	94,643	[1]	77,154	[1]
Operating Expenses									86,547		81,885		78,876
Other Expenses									328	[2]	2,707	[2]	2,678	[2]
Operating Income (Loss)									9,346		10,051		(4,400)
Income from Equity Method Investments									4,685		5,420		2,594
Pre-Tax Income (Loss) from Continuing Operations									14,031		15,471		(1,806)
Identifiable Segment Assets	$ 511,908				$ 486,893				$ 511,908		$ 486,893		$ 520,241

[1] Net revenues include Other Revenue, net, allocated to the segments as follows: For the Years Ended December 31, 2014 2013 2012Investment Banking (A)$(1,722) $3,979 $(3,019)Investment Management (B)(2,530) (1,116) (2,636)Total Other Revenue, net$(4,252) $2,863 $(5,655) (A)Investment Banking Other Revenue, net, includes interest expense on the Senior Notes of $4,470, $4,386 and $4,312 for the years ended December 31, 2014, 2013 and 2012, respectively, and changes in amounts due pursuant to the Company'stax receivable agreement of $5,524 for the year ended December 31, 2013.(B)Investment Management Other Revenue, net, includes interest expense on the Senior Notes of $3,770, $3,702 and $3,643 for the years ended December 31, 2014, 2013 and 2012, respectively, and changes in amounts due pursuant to theCompany's tax receivable agreement of $1,381 for the year ended December 31, 2013.

[2] Other Expenses are as follows: For the Years Ended December 31, 2014 2013 2012Investment Banking Amortization of LP Units and Certain Other Awards$3,399 $17,817 $18,601Other Acquisition Related Compensation Charges7,939 15,923 28,163Special Charges4,893 — 662Intangible Asset and Other Amortization2,494 — 3,348Professional Fees1,672 — —Acquisition and Transition Costs4,712 — —Total Investment Banking25,109 33,740 50,774Investment Management Amortization of LP Units and Certain Other Awards— 2,209 2,350Special Charges— 170 —Intangible Asset and Other Amortization328 328 328Total Investment Management328 2,707 2,678Total Other Expenses$25,437 $36,447 $53,452