Repurchase Agreements (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011 months	Dec. 31, 2010 months
Securities Sold Under Agreements To Repurchase [Line Items]
Number of counterparties in repurchase agreements	23	20
Weighted average contractual maturity of the repurchase agreements outstanding (in months)	0.8	0.9
Balance, repurchase agreement	$ 16,162,375	$ 8,681,060
Weighted Average Contractual Rate	0.37%	0.44%
Within 30 Days [Member]
Securities Sold Under Agreements To Repurchase [Line Items]
Balance, repurchase agreement	16,162,375	7,530,317
Weighted Average Contractual Rate	0.37%	0.38%
30 Days To 3 Months [Member]
Securities Sold Under Agreements To Repurchase [Line Items]
Balance, repurchase agreement		1,050,743
Weighted Average Contractual Rate		0.60%
3 Months To 36 Months [Member]
Securities Sold Under Agreements To Repurchase [Line Items]
Balance, repurchase agreement		$ 100,000
Weighted Average Contractual Rate		2.96%