Derivatives and Hedging Activities - Effect of Derivative Financial Instruments on Statement of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Derivative Instruments, Gain (Loss)
Realized gain (loss) on settlement, termination, expiration or exercise, net			$ (44,394)	$ (34,877)
Unrealized gain (loss), net			(104,546)	(133,863)
Gain (loss) on derivative instruments, net	$ (220,602)	$ (3,704)	(287,344)	(322,832)
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Realized gain (loss) on settlement, termination, expiration or exercise, net	(3,079)	(1,016)	(44,394)	(34,877)
Contractual interest expense	(46,785)	(50,446)	(138,404)	(154,092)
Unrealized gain (loss), net	(170,738)	47,758	(104,546)	(133,863)
Gain (loss) on derivative instruments, net	(220,602)	(3,704)	(287,344)	(322,832)
Not Designated as Hedging Instrument | Realized and unrealized credit derivative income (loss), net
Derivative Instruments, Gain (Loss)
Gain (loss) on derivative instruments, net	(3,944)	(33,722)	7,923	8,122
Not Designated as Hedging Instrument | CDS Contract | Realized and unrealized credit derivative income (loss), net
Derivative Instruments, Gain (Loss)
Gain (loss) on derivative instruments, net	(96)	(78)	648	(185)
Not Designated as Hedging Instrument | Embedded Credit Derivative | Realized and unrealized credit derivative income (loss), net
Derivative Instruments, Gain (Loss)
Gain (loss) on derivative instruments, net	(3,848)	(33,644)	7,275	8,307
Not Designated as Hedging Instrument | Interest Rate Swaps
Derivative Instruments, Gain (Loss)
Realized gain (loss) on settlement, termination, expiration or exercise, net	0	1,348	(31,881)	1,348
Contractual interest expense	(46,785)	(50,446)	(138,404)	(154,092)
Unrealized gain (loss), net	(173,817)	47,709	(113,151)	(146,116)
Gain (loss) on derivative instruments, net	(220,602)	(1,389)	(283,436)	(298,860)
Not Designated as Hedging Instrument | Interest Rate Swaptions
Derivative Instruments, Gain (Loss)
Realized gain (loss) on settlement, termination, expiration or exercise, net	(2,590)	0	(10,328)	(23,275)
Contractual interest expense	0	0	0	0
Unrealized gain (loss), net	2,589	(2,185)	8,594	13,596
Gain (loss) on derivative instruments, net	(1)	(2,185)	(1,734)	(9,679)
Not Designated as Hedging Instrument | TBA's
Derivative Instruments, Gain (Loss)
Realized gain (loss) on settlement, termination, expiration or exercise, net	0	(2,943)	(2,292)	(4,343)
Contractual interest expense	0	0	0	0
Unrealized gain (loss), net	0	368	558	(867)
Gain (loss) on derivative instruments, net	0	(2,575)	(1,734)	(5,210)
Not Designated as Hedging Instrument | Futures Contracts
Derivative Instruments, Gain (Loss)
Realized gain (loss) on settlement, termination, expiration or exercise, net	0	249	(943)	(8,937)
Contractual interest expense	0	0	0	0
Unrealized gain (loss), net	0	786	(90)	(1,556)
Gain (loss) on derivative instruments, net	0	1,035	(1,033)	(10,493)
Not Designated as Hedging Instrument | Currency Forward Contracts
Derivative Instruments, Gain (Loss)
Realized gain (loss) on settlement, termination, expiration or exercise, net	(489)	330	1,050	330
Contractual interest expense	0	0	0	0
Unrealized gain (loss), net	490	1,080	(457)	1,080
Gain (loss) on derivative instruments, net	$ 1	$ 1,410	$ 593	$ 1,410