Repurchase Agreements - Repurchase Agreements by Maturity Period (Details) - USD ($) $ in Thousands	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%
Maturity up to 30 days
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase Agreements	$ 7,366,962	$ 8,089,403
Weighted Average Contractual Rate	0.71%	0.55%
Maturity 31 to 60 Days
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase Agreements	$ 1,794,880	$ 2,487,174
Weighted Average Contractual Rate	0.70%	0.57%
Maturity 61 to 90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase Agreements	$ 112,017	$ 343,904
Weighted Average Contractual Rate	1.01%	0.71%
Maturity over 90 days
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase Agreements	$ 650,000	$ 650,000
Weighted Average Contractual Rate	0.81%	0.67%