Segment Information - Summary of Financial Information of Company's Segments (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
Revenues, net of interest expense:
Investment banking									$ 71,091	$ 115,255	$ 196,213
Institutional brokerage									45,442	56,182	53,738
Net investment income									5,433	17,774	6,920
Interest									31,774	13,067	1,790
Dividends and other									1,692	1,030	1,160
Total revenues									155,432	203,308	259,821
Interest expense									35,037	21,183	0
Revenues, net of interest expense	$ 23,464	$ 25,580	$ 44,256	$ 27,095	$ 28,572	$ 42,097	$ 57,098	$ 54,358	120,395	182,125	259,821
Operating expenses:
Variable									35,688	59,816	104,934
Fixed									100,465	104,949	97,617
Total non-interest expenses									136,153	164,765	202,551
(Loss) income from continuing operations before income taxes									(15,758)	17,360	57,270
Compensation and benefits:
Variable									18,889	42,713	90,746
Fixed									58,574	61,098	53,974
Total									77,463	103,811	144,720
Total assets	933,416				1,035,097				933,416	1,035,097	410,572
Capital Markets [Member]
Revenues, net of interest expense:
Investment banking									71,091	115,255	196,213
Institutional brokerage									45,442	56,182	53,738
Net investment income									0	0	0
Interest									31,745	12,945	1,050
Dividends and other									1,103	549	432
Total revenues									149,381	184,931	251,433
Interest expense									23,877	9,505	0
Revenues, net of interest expense									125,504	175,426	251,433
Operating expenses:
Variable									35,651	59,500	103,755
Fixed									98,331	102,426	96,121
Total non-interest expenses									133,982	161,926	199,876
(Loss) income from continuing operations before income taxes									(8,478)	13,500	51,557
Compensation and benefits:
Variable									18,889	42,408	89,574
Fixed									57,165	59,639	53,013
Total									76,054	102,047	142,587
Total assets	834,205				830,938				834,205	830,938	308,870
Principal Investing [Member]
Revenues, net of interest expense:
Investment banking									0	0	0
Institutional brokerage									0	0	0
Net investment income									5,433	17,774	6,920
Interest									29	122	740
Dividends and other									589	481	728
Total revenues									6,051	18,377	8,388
Interest expense									11,160	11,678	0
Revenues, net of interest expense									(5,109)	6,699	8,388
Operating expenses:
Variable									37	316	1,179
Fixed									2,134	2,523	1,496
Total non-interest expenses									2,171	2,839	2,675
(Loss) income from continuing operations before income taxes									(7,280)	3,860	5,713
Compensation and benefits:
Variable									0	305	1,172
Fixed									1,409	1,459	961
Total									1,409	1,764	2,133
Total assets	$ 99,211				$ 204,159				$ 99,211	$ 204,159	$ 101,702