Repurchase Agreements - Contractual Repricing on Repurchase Agreements (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase Agreements	$ 9,923,859	$ 11,570,481
Weighted Average Contractual Rate	0.72%	0.57%
Weighted Average Maturity in days	32 days	38 days
Haircut for Repurchase Agreements	5.21%	4.79%
Agency Securities
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase Agreements	$ 9,722,888	$ 11,570,481
Weighted Average Contractual Rate	0.69%	0.57%
Weighted Average Maturity in days	32 days	38 days
Haircut for Repurchase Agreements	4.80%	4.79%
Non-Agency Securities
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase Agreements	$ 200,971
Weighted Average Contractual Rate	2.15%
Weighted Average Maturity in days	31 days
Haircut for Repurchase Agreements	24.83%