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
Mar. 31, 2012
Dec. 31, 2011
Derivative [Line Items]    
Derivative, Notional Amount $ 6,035,000 $ 4,385,000
Derivative, Average Fixed Interest Rate 0.8521% 0.9523%
Derivative, Average Variable Interest Rate 0.5209% 0.3611%
Derivative, Average Remaining Maturity 2.283 2.406
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 0.5634% 0.315%
Derivative, Average Remaining Maturity .725 .975
More than 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 0.5131% 0.368%
Derivative, Average Remaining Maturity 1.308 1.546
More than 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 0.5529% 0.3801%
Derivative, Average Remaining Maturity 2.316 2.719
More than Three and within Four Years from Balance Sheet Date [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 1,670,000 820,000
Derivative, Average Fixed Interest Rate 1.1355% 1.5747%
Derivative, Average Variable Interest Rate 0.5041% 0.329%
Derivative, Average Remaining Maturity 3.094 3.524
More than Four and within Five Years from Balance Sheet Date [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount $ 390,000 $ 240,000
Derivative, Average Fixed Interest Rate 1.3421% 2.1556%
Derivative, Average Variable Interest Rate 0.4981% 0.3158%
Derivative, Average Remaining Maturity 4.459 4.322