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 $)
Mar. 31, 2012
Dec. 31, 2011
Repurchase Agreement Counterparty [Line Items]    
Assets Sold under Agreements to Repurchase, Repurchase Liability $ 8,693,756,000 $ 6,660,148,000
Repurchase Agreement Counterparty, JP Morgan Chase [Member]
    
Repurchase Agreement Counterparty [Line Items]    
Repurchase Agreement Counterparty, Amount at Risk 265,317,000 184,046,000
Repurchase Agreement Counterparty, Percent of Equity at Risk 0.13 0.14
Repurchase Agreement Counterparty, Weighted Average Maturity of Agreements 113.6 70.0
Assets Sold under Agreements to Repurchase, Repurchase Liability 1,608,354,000 1,250,629,000
Repurchase Agreement Counterparty, Credit Suisse [Member]
    
Repurchase Agreement Counterparty [Line Items]    
Repurchase Agreement Counterparty, Amount at Risk 237,801,000 46,006,000
Repurchase Agreement Counterparty, Percent of Equity at Risk 0.11 0.04
Repurchase Agreement Counterparty, Weighted Average Maturity of Agreements 20.0 14.1
Assets Sold under Agreements to Repurchase, Repurchase Liability 280,498,000 95,691,000
Repurchase Agreement Counterparty, All Other Counterparties [Member]
    
Repurchase Agreement Counterparty [Line Items]    
Repurchase Agreement Counterparty, Amount at Risk 768,859,000 567,440,000
Repurchase Agreement Counterparty, Percent of Equity at Risk 0.37 0.45
Repurchase Agreement Counterparty, Weighted Average Maturity of Agreements 74.0 75.1
Assets Sold under Agreements to Repurchase, Repurchase Liability 5,803,654,000 4,312,578,000
Repurchase Agreement Counterparty, Excluding Repurchase Agreements Collateralized by U.S. Treasuries [Member]
    
Repurchase Agreement Counterparty [Line Items]    
Repurchase Agreement Counterparty, Amount at Risk 1,271,977,000 797,492,000
Assets Sold under Agreements to Repurchase, Repurchase Liability $ 7,692,506,000 $ 5,658,898,000