Derivatives and Hedging Activities - Effect of Derivative Financial Instruments on Statement of Operations (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Derivative Instruments, Gain (Loss)
Realized gain (loss) on settlement, termination, expiration or exercise, net			$ (63,569,000)	$ (41,315,000)
Unrealized gain (loss), net			(211,261,000)	66,192,000
Gain (loss) on derivative instruments, net	$ (90,363,000)	$ 56,003,000	(328,906,000)	(66,742,000)
Interest Rate Swaptions
Derivative Instruments, Gain (Loss)
Unrealized gain (loss), net	0	2,300,000	1,500,000	6,000,000
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Realized gain (loss) on settlement, termination, expiration or exercise, net	(20,584,000)	(15,212,000)	(63,569,000)	(41,315,000)
Contractual interest expense	(24,985,000)	(46,011,000)	(54,076,000)	(91,619,000)
Unrealized gain (loss), net	(44,794,000)	117,226,000	(211,261,000)	66,192,000
Gain (loss) on derivative instruments, net	(90,363,000)	56,003,000	(328,906,000)	(66,742,000)
Not Designated as Hedging Instrument | Realized and unrealized credit derivative income (loss), net
Derivative Instruments, Gain (Loss)
Gain (loss) on derivative instruments, net	11,116,000	(4,109,000)	13,212,000	11,867,000
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	0	806,000	0	744,000
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	11,116,000	(4,915,000)	13,212,000	11,123,000
Not Designated as Hedging Instrument | Interest Rate Swaps
Derivative Instruments, Gain (Loss)
Realized gain (loss) on settlement, termination, expiration or exercise, net	(20,105,000)	(12,826,000)	(64,000,000)	(31,881,000)
Contractual interest expense	(24,985,000)	(46,011,000)	(54,076,000)	(91,619,000)
Unrealized gain (loss), net	(49,711,000)	116,623,000	(216,382,000)	60,666,000
Gain (loss) on derivative instruments, net	(94,801,000)	57,786,000	(334,458,000)	(62,834,000)
Not Designated as Hedging Instrument | Interest Rate Swaptions
Derivative Instruments, Gain (Loss)
Realized gain (loss) on settlement, termination, expiration or exercise, net		(3,050,000)	(1,485,000)	(7,738,000)
Contractual interest expense			0	0
Unrealized gain (loss), net		2,326,000	1,485,000	6,005,000
Gain (loss) on derivative instruments, net		(724,000)	0	(1,733,000)
Not Designated as Hedging Instrument | Currency Forward Contracts
Derivative Instruments, Gain (Loss)
Realized gain (loss) on settlement, termination, expiration or exercise, net	(479,000)	664,000	1,916,000	1,539,000
Contractual interest expense	0		0	0
Unrealized gain (loss), net	4,917,000	(1,723,000)	3,636,000	(947,000)
Gain (loss) on derivative instruments, net	$ 4,438,000	$ (1,059,000)	$ 5,552,000	592,000
Not Designated as Hedging Instrument | TBA's
Derivative Instruments, Gain (Loss)
Realized gain (loss) on settlement, termination, expiration or exercise, net				(2,292,000)
Contractual interest expense				0
Unrealized gain (loss), net				558,000
Gain (loss) on derivative instruments, net				(1,734,000)
Not Designated as Hedging Instrument | Futures Contracts
Derivative Instruments, Gain (Loss)
Realized gain (loss) on settlement, termination, expiration or exercise, net				(943,000)
Contractual interest expense				0
Unrealized gain (loss), net				(90,000)
Gain (loss) on derivative instruments, net				$ (1,033,000)