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
Sep. 30, 2012
Dec. 31, 2011
Repurchase Agreement Counterparty [Line Items]    
Assets Sold under Agreements to Repurchase, Repurchase Liability $ 14,034,327 $ 6,660,148
Payables to Broker-Dealers and Clearing Organizations 15,749 45,565
Repurchase Agreement Counterparty, JP Morgan Chase [Member]
   
Repurchase Agreement Counterparty [Line Items]    
Assets Sold under Agreements to Repurchase, Repurchase Liability 1,834,277 1,250,629
Repurchase Agreement Counterparty, Amount at Risk 321,627 184,046
Repurchase Agreement Counterparty, Percent of Equity at Risk 9.00% 14.00%
Repurchase Agreement Counterparty, Weighted Average Maturity 145.5 69.98
Repurchase Agreement Counterparty, All Other Counterparties [Member]
   
Repurchase Agreement Counterparty [Line Items]    
Assets Sold under Agreements to Repurchase, Repurchase Liability 11,202,550 4,408,269
Repurchase Agreement Counterparty, Amount at Risk 1,470,542 613,446
Repurchase Agreement Counterparty, Percent of Equity at Risk 43.00% 48.00%
Repurchase Agreement Counterparty, Weighted Average Maturity 119.5 73.78
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 13,036,827 5,658,898
Repurchase Agreement Counterparty, Amount at Risk $ 1,792,169 $ 797,492