Quarterly report pursuant to Section 13 or 15(d)

Repurchase Agreements Schedule of Repurchase Agreements by Collateral Type (Details)

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Repurchase Agreements Schedule of Repurchase Agreements by Collateral Type (Details) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
Assets Sold under Agreements to Repurchase [Line Items]    
Assets Sold under Agreements to Repurchase, Repurchase Liability $ 14,034,327 $ 6,660,148
Assets Sold under Agreements to Repurchase, Weighted Average Borrowing Rate, Excluding U.S. Treasuries and Interest Rate Swaps 0.71% 0.78%
Assets Sold under Agreements to Repurchase, Weighted Average Remaining Maturity, Excluding U.S. Treasuries and Interest Rate Swaps 123 73
Assets Sold under Agreements to Repurchase, Interest Rate 0.68% 0.68%
US Treasury Securities [Member]
    
Assets Sold under Agreements to Repurchase [Line Items]    
Assets Sold under Agreements to Repurchase, Repurchase Liability 997,500 1,001,250
Assets Sold under Agreements to Repurchase, Interest Rate 0.35% 0.12%
US Government Agencies Debt Securities [Member]
    
Assets Sold under Agreements to Repurchase [Line Items]    
Assets Sold under Agreements to Repurchase, Repurchase Liability 11,671,026 4,804,533
Assets Sold under Agreements to Repurchase, Interest Rate 0.52% 0.50%
Mortgage-backed Securities, Issued by Private Enterprises [Member]
    
Assets Sold under Agreements to Repurchase [Line Items]    
Assets Sold under Agreements to Repurchase, Repurchase Liability 1,111,954 731,014
Assets Sold under Agreements to Repurchase, Interest Rate 2.56% 2.61%
Inverse Interest-Only Securities [Member]
    
Assets Sold under Agreements to Repurchase [Line Items]    
Assets Sold under Agreements to Repurchase, Repurchase Liability 241,035 118,032
Assets Sold under Agreements to Repurchase, Interest Rate 1.16% 0.97%
Loans Held-for-sale, Mortgages [Member]
    
Assets Sold under Agreements to Repurchase [Line Items]    
Assets Sold under Agreements to Repurchase, Repurchase Liability $ 12,812 $ 5,319
Assets Sold under Agreements to Repurchase, Interest Rate 2.46% 3.20%