Contractual Maturity Information Regarding Repurchase Agreements (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements, Carrying Value	$ 11,419,354	$ 13,443,883
Weighted Average Contractual Rate	0.67%	0.65%
Within 30 days
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements, Carrying Value	$ 10,669,354	$ 12,693,883
Weighted Average Contractual Rate	0.66%	0.64%
30 days to 3 months
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements, Carrying Value	$ 500,000
Weighted Average Contractual Rate	0.90%	0.00%
3 months to 36 months
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements, Carrying Value	$ 250,000	$ 750,000
Weighted Average Contractual Rate	0.79%	0.75%