Contractual Maturity Information Regarding Repurchase Agreements (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements	$ 14,240,809	$ 15,759,831
Weighted Average Contractual Rate	0.46%	0.36%
Within 30 days
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements	$ 12,990,809	$ 13,770,099
Weighted Average Contractual Rate	0.44%	0.35%
30 days to 3 months
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements	$ 250,000	$ 1,489,732
Weighted Average Contractual Rate	0.52%	0.36%
3 months to 36 months
Assets Sold Under Agreements To Repurchase [Line Items]
Repurchase agreements	$ 1,000,000	$ 500,000
Weighted Average Contractual Rate	0.59%	0.53%