Derivative Instruments and Hedging Activities - Activity of Derivative Instruments Not Designated as Hedges (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Interest rate swaps
Derivative Instruments and Hedging Activities Disclosures [Roll Forward]
Beginning balance	$ 345,500
Additions	50,000
Settlement, Expiration or Exercise	0
Ending balance	$ 395,500
Trading | TBA securities
Derivative Instruments and Hedging Activities Disclosures [Roll Forward]
Beginning balance		$ 149,000
Additions		1,011,000
Settlement, Expiration or Exercise		(994,000)
Ending balance		166,000
Trading | U.S. Treasury futures
Derivative Instruments and Hedging Activities Disclosures [Roll Forward]
Beginning balance		17,100
Additions		108,000
Settlement, Expiration or Exercise		(111,800)
Ending balance		13,300
Trading | Interest rate swap futures
Derivative Instruments and Hedging Activities Disclosures [Roll Forward]
Beginning balance		(151,700)
Additions		315,200
Settlement, Expiration or Exercise		(259,400)
Ending balance		(95,900)
Trading | Eurodollar futures
Derivative Instruments and Hedging Activities Disclosures [Roll Forward]
Beginning balance		(2,575,000)
Additions		4,790,000
Settlement, Expiration or Exercise		(4,001,000)
Ending balance		(1,786,000)
Trading | Swaptions
Derivative Instruments and Hedging Activities Disclosures [Roll Forward]
Beginning balance		154,000
Additions		0
Settlement, Expiration or Exercise		0
Ending balance		154,000
Trading | Interest rate swaps
Derivative Instruments and Hedging Activities Disclosures [Roll Forward]
Beginning balance		15,000
Additions		0
Settlement, Expiration or Exercise		0
Ending balance		$ 15,000