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, 2016	Dec. 31, 2015
TBA Securities
Derivative Instruments and Hedging Activities Disclosures [Roll Forward]
Beginning Balance	$ 222,000	$ 273,000
Additions	4,070,000	3,801,000
Settlement, Expiration or Exercise	(4,143,000)	(3,852,000)
Ending Balance	149,000	222,000
U.S. Treasury futures
Derivative Instruments and Hedging Activities Disclosures [Roll Forward]
Beginning Balance	0	2,300
Additions	201,900	150,200
Settlement, Expiration or Exercise	(184,800)	(152,500)
Ending Balance	17,100	0
Interest rate swap futures
Derivative Instruments and Hedging Activities Disclosures [Roll Forward]
Beginning Balance	(137,200)	(190,100)
Additions	868,800	1,165,200
Settlement, Expiration or Exercise	(883,300)	(1,112,300)
Ending Balance	(151,700)	(137,200)
Eurodollar futures
Derivative Instruments and Hedging Activities Disclosures [Roll Forward]
Beginning Balance	(2,769,000)	(2,961,000)
Additions	6,323,000	2,925,000
Settlement, Expiration or Exercise	(6,129,000)	(2,733,000)
Ending Balance	(2,575,000)	(2,769,000)
Options on U.S. Treasury futures
Derivative Instruments and Hedging Activities Disclosures [Roll Forward]
Beginning Balance	28,000	21,000
Additions	111,000	375,000
Settlement, Expiration or Exercise	(139,000)	(368,000)
Ending Balance	0	28,000
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)	(30,000)
Ending Balance	154,000	159,000
Interest rate swaps
Derivative Instruments and Hedging Activities Disclosures [Roll Forward]
Beginning Balance	10,000	10,000
Additions	5,000	0
Settlement, Expiration or Exercise	0	0
Ending Balance	$ 15,000	$ 10,000