Summary of Statement of Operations of Deconsolidated VIEs (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended			0 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	Dec. 19, 2014
Revenue:
Investment interest income									$ 133,419	$ 134,447	$ 117,054
Investment interest expense									(30,310)	(30,595)	(32,909)
Net interest margin									103,109	103,852	84,145
Fee and other income									24,280	28,799	12,767
Total revenue	73,857	75,293	73,256	67,308	67,607	62,395	58,622	58,251	289,714	246,875	200,784
Expenses:
Compensation expense									28,168	26,802	23,182
General and administrative expense									17,653	14,496	14,709
Depreciation and amortization expense									56,784	36,093	31,337
Total expenses									247,482	181,972	170,236
Operating income									42,232	64,903	30,548
Other income (expense)									(21,398)	(5,233)	(1,789)
Gains (losses) on assets									(5,370)	(2,266)	2,529
Gains (losses) on deconsolidation of VIEs									(215,804)
Net gain on sale of Collateral Management contracts									4,549
Change in fair value of financial instruments	(39,319)	(10,223)	(25,071)	(24,139)	(143,990)	(24,659)	(76,020)	(99,757)	(98,752)	(344,426)	(201,787)
Income (loss) before taxes									(292,122)	(288,297)	(168,925)
Income tax benefit (provision)									2,147	2,933	584
Net income (loss) allocable to common shares	(255,001)	(23,266)	(25,650)	(14,587)	(134,493)	(17,106)	(65,877)	(90,532)	(318,504)	(308,008)	(182,805)
Deconsolidation of VIEs
Revenue:
Investment interest income									39,551	48,937	55,818
Investment interest expense									(16,765)	(18,805)	(19,835)
Net interest margin									22,786	30,132	35,983
Fee and other income									980	1,363	1,478
Total revenue									23,766	31,495	37,461
Expenses:
Compensation expense									1,736	1,672	1,212
General and administrative expense									1,440	1,601	1,750
Depreciation and amortization expense									136	137	208
Total expenses									3,312	3,410	3,170
Operating income									20,454	28,085	34,291
Other income (expense)									(21,500)		(1,619)
Gains (losses) on assets									(7,712)	(90)
Gains (losses) on deconsolidation of VIEs									(215,804)			(215,804)
Net gain on sale of Collateral Management contracts									4,549			4,549
Change in fair value of financial instruments									(106,593)	(322,301)	(192,404)
Income (loss) before taxes									(326,606)	(294,306)	(159,732)
Income tax benefit (provision)									(496)	(714)	(182)
Net income (loss) allocable to common shares									$ (327,102)	$ (295,020)	$ (159,914)