Quarterly report pursuant to Section 13 or 15(d)

Repurchase Agreements Schedule of Repurchase Agreement Counterparties with Whom Repurchase Agreements Exceed 10 Percent of Stockholders' Equity (Details)

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Repurchase Agreements Schedule of Repurchase Agreement Counterparties with Whom Repurchase Agreements Exceed 10 Percent of Stockholders' Equity (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Repurchase Agreement Counterparty [Line Items]    
Assets Sold under Agreements to Repurchase, Repurchase Liability $ 13,444,565 $ 12,624,510
Repurchase Agreement Counterparty, JP Morgan Chase [Member]
   
Repurchase Agreement Counterparty [Line Items]    
Assets Sold under Agreements to Repurchase, Repurchase Liability 1,327,938 1,382,348
Repurchase Agreement Counterparty, Amount at Risk 266,429 281,717
Repurchase Agreement Counterparty, Percent of Equity at Risk 7.00% 8.00%
Repurchase Agreement Counterparty, Weighted Average Maturity 77.4 70.8
Repurchase Agreement Counterparty, All Other Counterparties [Member]
   
Repurchase Agreement Counterparty [Line Items]    
Assets Sold under Agreements to Repurchase, Repurchase Liability 11,111,627 10,244,662
Repurchase Agreement Counterparty, Amount at Risk 1,319,993 1,379,409
Repurchase Agreement Counterparty, Percent of Equity at Risk 32.00% 40.00%
Repurchase Agreement Counterparty, Weighted Average Maturity 81.8 86.4
Repurchase Agreement Counterparty, Excluding Repurchase Agreements Collateralized by U.S. Treasuries [Member]
   
Repurchase Agreement Counterparty [Line Items]    
Assets Sold under Agreements to Repurchase, Repurchase Liability 12,439,565 11,627,010
Repurchase Agreement Counterparty, Amount at Risk 1,586,422 1,661,126
Payables to Broker-Dealers and Clearing Organizations $ 291,519 $ 291,675
Number of Repurchase Agreement Counterparties with Whom Repurchase Agreements Are Less Than 10 Percent of Stockholders' Equity 19 21