Quarterly report pursuant to Section 13 or 15(d)

Derivative Instruments and Hedging Activities Schedule of Interest Rate Swaps Associated with Repurchase Agreements (Details)

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Derivative Instruments and Hedging Activities Schedule of Interest Rate Swaps Associated with Repurchase Agreements (Details) (Swap [Member], USD $)
In Thousands, unless otherwise specified
Jun. 30, 2012
Dec. 31, 2011
Derivative [Line Items]    
Derivative, Notional Amount $ 8,135,000 $ 4,385,000
Derivative, Average Fixed Interest Rate 0.8695% 0.9523%
Derivative, Average Variable Interest Rate 0.4839% 0.3611%
Derivative, Weighted Average Remaining Maturity 2.553 2.406
Derivative Maturity Within One Year From Balance Sheet Date [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 25,000 25,000
Derivative, Average Fixed Interest Rate 0.8675% 0.8675%
Derivative, Average Variable Interest Rate 0.5215% 0.315%
Derivative, Weighted Average Remaining Maturity .475 .975
Derivative Maturity Over One And Within Two Years From Balance Sheet Date [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 2,275,000 2,025,000
Derivative, Average Fixed Interest Rate 0.713% 0.7366%
Derivative, Average Variable Interest Rate 0.4995% 0.368%
Derivative, Weighted Average Remaining Maturity 1.058 1.546
Derivative Maturity Over Two And Within Three Years From Balance Sheet Date [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 1,675,000 1,275,000
Derivative, Average Fixed Interest Rate 0.644% 0.6699%
Derivative, Average Variable Interest Rate 0.5173% 0.3801%
Derivative, Weighted Average Remaining Maturity 2.066 2.719
Derivative Maturity Over Three And Within Four Years From Balance Sheet Date [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 2,070,000 820,000
Derivative, Average Fixed Interest Rate 1.0388% 1.5747%
Derivative, Average Variable Interest Rate 0.4469% 0.329%
Derivative, Weighted Average Remaining Maturity 2.874 3.524
Derivative Maturity Over Four And Within Five Years From Balance Sheet Date [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount $ 2,090,000 $ 240,000
Derivative, Average Fixed Interest Rate 1.0527% 2.1556%
Derivative, Average Variable Interest Rate 0.4761% 0.3158%
Derivative, Weighted Average Remaining Maturity 4.276 4.322