Derivative Instruments and Hedging Activities - Derivative Instruments Not Designated as Hedges (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Settlement, Expiration or Exercise		$ 0
Not Designated as Hedging Instrument
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Settlement, Expiration or Exercise	$ 0
Not Designated as Hedging Instrument | TBA securities
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Beginning balance	222,000	273,000
Additions	1,065,000	1,171,000
Settlement, Expiration or Exercise	(1,006,000)	(1,123,000)
Ending balance	281,000	321,000
Not Designated as Hedging Instrument | U.S. Treasury futures
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Beginning balance	0	2,300
Additions	70,600	91,500
Settlement, Expiration or Exercise	(35,300)	(64,200)
Ending balance	35,300	29,600
Not Designated as Hedging Instrument | Interest rate swap futures
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Beginning balance	(137,200)	(190,100)
Additions	316,500	318,500
Settlement, Expiration or Exercise	(272,900)	(336,400)
Ending balance	(93,600)	(208,000)
Not Designated as Hedging Instrument | Eurodollar futures
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Beginning balance	(2,769,000)	(2,961,000)
Additions	676,000	471,000
Settlement, Expiration or Exercise	(1,484,000)	(388,000)
Ending balance	(3,577,000)	(2,878,000)
Not Designated as Hedging Instrument | Options on U.S. Treasury futures
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Beginning balance	28,000	21,000
Additions	29,000	152,000
Settlement, Expiration or Exercise	(35,000)	(122,000)
Ending balance	22,000	51,000
Not Designated as Hedging Instrument | Swaptions
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Beginning balance	159,000	180,000
Additions	0	5,000
Settlement, Expiration or Exercise	(5,000)	0
Ending balance	154,000	185,000
Not Designated as Hedging Instrument | Interest rate swaps
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Beginning balance	10,000	10,000
Additions	5,000	0
Ending balance	$ 15,000	$ 10,000