Derivative Instruments and Hedging Activities - Derivative Instruments Not Designated as Hedges (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
TBA securities
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Beginning balance	$ 273,000	$ 188,000
Additions	3,801,000	2,575,000
Settlement, Expiration or Exercise	(3,852,000)	(2,490,000)
Ending Balance	222,000	273,000
US Treasury Futures
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Beginning balance	2,300	(11,900)
Additions	150,200	130,500
Settlement, Expiration or Exercise	(152,500)	(116,300)
Ending Balance	0	2,300
Interest Rate Swap Future
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Beginning balance	(190,100)	(242,700)
Additions	1,165,200	996,500
Settlement, Expiration or Exercise	(1,112,300)	(943,900)
Ending Balance	(137,200)	(190,100)
Eurodollar Future
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Beginning balance	(2,961,000)	(3,360,000)
Additions	2,925,000	3,134,000
Settlement, Expiration or Exercise	(2,733,000)	(2,735,000)
Ending Balance	(2,769,000)	(2,961,000)
Options on US Treasury Futures
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Beginning balance	21,000	40,000
Additions	375,000	48,200
Settlement, Expiration or Exercise	(368,000)	(67,200)
Ending Balance	28,000	21,000
Swaption
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Beginning balance	180,000	100,000
Additions	9,000	150,000
Settlement, Expiration or Exercise	(30,000)	(70,000)
Ending Balance	159,000	180,000
Interest Rate Swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Beginning balance	10,000	0
Additions	0	10,000
Ending Balance	$ 10,000	$ 10,000