Annual report pursuant to Section 13 and 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 $)
Dec. 31, 2011
Dec. 31, 2010
Repurchase Agreement Counterparty [Line Items]    
Repurchase Agreement Counterparty, Amount at Risk $ 800,443,000 $ 177,186,000
Assets Sold under Agreements to Repurchase, Repurchase Liability 6,660,148,000 1,169,803,000
Repurchase Agreement Counterparty, JP Morgan Chase [Member]
    
Repurchase Agreement Counterparty [Line Items]    
Repurchase Agreement Counterparty, Amount at Risk 186,997,000 8,687,000
Repurchase Agreement Counterparty, Percent of Equity at Risk 0.15 0.02
Repurchase Agreement Counterparty, Weighted Average Maturity of Agreements 70.0 44.7
Assets Sold under Agreements to Repurchase, Repurchase Liability 2,251,879,000 289,321,000
Repurchase Agreement Counterparty, Barclays Capital [Member]
    
Repurchase Agreement Counterparty [Line Items]    
Repurchase Agreement Counterparty, Amount at Risk 84,391,000 45,060,000
Repurchase Agreement Counterparty, Percent of Equity at Risk 0.07 0.12
Repurchase Agreement Counterparty, Weighted Average Maturity of Agreements 56.7 53.0
Assets Sold under Agreements to Repurchase, Repurchase Liability 242,608,000 168,291,000
Repurchase Agreement Counterparty, All Other Counterparties [Member]
    
Repurchase Agreement Counterparty [Line Items]    
Repurchase Agreement Counterparty, Amount at Risk 529,055,000 123,439,000
Repurchase Agreement Counterparty, Percent of Equity at Risk 0.42 0.32
Repurchase Agreement Counterparty, Weighted Average Maturity of Agreements 74.9 104.3
Assets Sold under Agreements to Repurchase, Repurchase Liability $ 4,165,661,000 $ 712,191,000