Segment Operating Results (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Net Revenues	$ 408,243	$ 308,951	$ 268,096	$ 237,983	$ 321,888	$ 227,161	$ 217,696	$ 149,113	$ 1,223,273	$ 915,858	$ 765,428
Operating Expenses									946,532	719,474	598,806
Other Expenses									148,071	25,437	36,447
Income Before Income from Equity Method Investments and Income Taxes	74,663	11,898	31,111	10,998	67,852	39,346	43,035	20,714	128,670	170,947	130,175
Income from Equity Method Investments	2,016	929	1,998	1,107	1,799	1,102	2,038	241	6,050	5,180	8,326
Pre-Tax Income (Loss) from Continuing Operations	76,679	$ 12,827	$ 33,109	$ 12,105	69,651	$ 40,448	$ 45,073	$ 20,955	134,720	176,127	138,501
Identifiable Segment Assets	1,479,171				1,446,556				1,479,171	1,446,556	1,180,783
Investment Banking [Member]
Segment Reporting Information [Line Items]
Net Revenues									1,130,915	819,637	670,785
Operating Expenses									869,301	632,927	516,921
Other Expenses									108,739	25,109	33,740
Income Before Income from Equity Method Investments and Income Taxes									152,875	161,601	120,124
Income from Equity Method Investments									978	495	2,906
Pre-Tax Income (Loss) from Continuing Operations									153,853	162,096	123,030
Identifiable Segment Assets	1,097,373				934,648				1,097,373	934,648	693,890
Investment Management [Member]
Segment Reporting Information [Line Items]
Net Revenues									92,358	96,221	94,643
Operating Expenses									77,231	86,547	81,885
Other Expenses									39,332	328	2,707
Income Before Income from Equity Method Investments and Income Taxes									(24,205)	9,346	10,051
Income from Equity Method Investments									5,072	4,685	5,420
Pre-Tax Income (Loss) from Continuing Operations									(19,133)	14,031	15,471
Identifiable Segment Assets	$ 381,798				$ 511,908				$ 381,798	$ 511,908	$ 486,893