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
Mar. 31, 2013
Dec. 31, 2012
Assets Sold under Agreements to Repurchase [Line Items]    
Assets Sold under Agreements to Repurchase, Repurchase Liability $ 13,444,565 $ 12,624,510
Assets Sold under Agreements to Repurchase, Weighted Average Borrowing Rate, Excluding U.S. Treasuries and Interest Rate Swaps 0.70% 0.76%
Assets Sold under Agreements to Repurchase, Weighted Average Remaining Maturity, Excluding U.S. Treasuries and Interest Rate Swaps 82 85
Assets Sold under Agreements to Repurchase, Interest Rate 0.66% 0.72%
US Treasury Securities [Member]
    
Assets Sold under Agreements to Repurchase [Line Items]    
Assets Sold under Agreements to Repurchase, Repurchase Liability 1,005,000 997,500
Assets Sold under Agreements to Repurchase, Interest Rate 0.23% 0.30%
US Government Agencies Debt Securities [Member]
    
Assets Sold under Agreements to Repurchase [Line Items]    
Assets Sold under Agreements to Repurchase, Repurchase Liability 10,897,155 10,171,385
Assets Sold under Agreements to Repurchase, Interest Rate 0.49% 0.54%
Mortgage-backed Securities, Issued by Private Enterprises [Member]
    
Assets Sold under Agreements to Repurchase [Line Items]    
Assets Sold under Agreements to Repurchase, Repurchase Liability 1,289,846 1,177,675
Assets Sold under Agreements to Repurchase, Interest Rate 2.35% 2.50%
Inverse Interest-Only Securities [Member]
    
Assets Sold under Agreements to Repurchase [Line Items]    
Assets Sold under Agreements to Repurchase, Repurchase Liability 229,488 228,241
Assets Sold under Agreements to Repurchase, Interest Rate 1.10% 1.16%
Loans Held-for-sale, Mortgages [Member]
    
Assets Sold under Agreements to Repurchase [Line Items]    
Assets Sold under Agreements to Repurchase, Repurchase Liability $ 23,076 $ 49,709
Assets Sold under Agreements to Repurchase, Interest Rate 2.45% 2.46%