Derivative Instruments and Hedging Activities - Derivative Instruments Not Designated as Hedges (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Derivative Instruments and Hedging Activities Disclosures [Roll Forward]
Settlement, Expiration or Exercise		$ 0
Not Designated as Hedging Instrument
Derivative Instruments and Hedging Activities Disclosures [Roll Forward]
Settlement, Expiration or Exercise	$ 0
Not Designated as Hedging Instrument | TBA securities
Derivative Instruments and Hedging Activities Disclosures [Roll Forward]
Beginning balance	222,000	273,000
Additions	1,952,000	2,176,000
Settlement, Expiration or Exercise	(1,893,000)	(2,153,000)
Ending balance	281,000	296,000
Not Designated as Hedging Instrument | U.S. Treasury futures
Derivative Instruments and Hedging Activities Disclosures [Roll Forward]
Beginning balance	0	2,300
Additions	117,700	123,600
Settlement, Expiration or Exercise	(78,000)	(135,400)
Ending balance	39,700	(9,500)
Not Designated as Hedging Instrument | Interest rate swap futures
Derivative Instruments and Hedging Activities Disclosures [Roll Forward]
Beginning balance	(137,200)	(190,100)
Additions	546,600	597,800
Settlement, Expiration or Exercise	(477,300)	(605,000)
Ending balance	(67,900)	(197,300)
Not Designated as Hedging Instrument | Eurodollar futures
Derivative Instruments and Hedging Activities Disclosures [Roll Forward]
Beginning balance	(2,769,000)	(2,961,000)
Additions	1,640,000	1,463,000
Settlement, Expiration or Exercise	(3,095,000)	(1,453,000)
Ending balance	(4,224,000)	(2,951,000)
Not Designated as Hedging Instrument | Options on U.S. Treasury futures
Derivative Instruments and Hedging Activities Disclosures [Roll Forward]
Beginning balance	28,000	21,000
Additions	66,000	187,000
Settlement, Expiration or Exercise	(64,000)	(201,000)
Ending balance	30,000	7,000
Not Designated as Hedging Instrument | Swaptions
Derivative Instruments and Hedging Activities Disclosures [Roll Forward]
Beginning balance	159,000	180,000
Additions	0	9,000
Settlement, Expiration or Exercise	(5,000)	0
Ending balance	154,000	189,000
Not Designated as Hedging Instrument | Interest rate swaps
Derivative Instruments and Hedging Activities Disclosures [Roll Forward]
Beginning balance	10,000	10,000
Additions	5,000	0
Ending balance	$ 15,000	$ 10,000