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, 2013
Dec. 31, 2012
Derivative [Line Items]    
Derivative, Notional Amount $ 15,685,000 $ 12,570,000
Derivative, Average Fixed Interest Rate 0.709% 0.85%
Derivative, Average Variable Interest Rate 0.298% 0.315%
Derivative, Weighted Average Remaining Maturity 3.360 2.847
Derivative Maturity Within One Year From Balance Sheet Date [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 500,000 2,275,000
Derivative, Average Fixed Interest Rate 0.523% 0.713%
Derivative, Average Variable Interest Rate 0.288% 0.315%
Derivative, Weighted Average Remaining Maturity .400 .558
Derivative Maturity Over One And Within Two Years From Balance Sheet Date [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 900,000 1,675,000
Derivative, Average Fixed Interest Rate 0.316% 0.644%
Derivative, Average Variable Interest Rate 0.304% 0.311%
Derivative, Weighted Average Remaining Maturity .792 1.566
Derivative Maturity Over Two And Within Three Years From Balance Sheet Date [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 4,000,000 2,770,000
Derivative, Average Fixed Interest Rate 0.386% 0.908%
Derivative, Average Variable Interest Rate 0.305% 0.313%
Derivative, Weighted Average Remaining Maturity 1.784 2.427
Derivative Maturity Over Three And Within Four Years From Balance Sheet Date [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 2,550,000 1,940,000
Derivative, Average Fixed Interest Rate 0.583% 0.874%
Derivative, Average Variable Interest Rate 0.298% 0.323%
Derivative, Weighted Average Remaining Maturity 2.918 3.456
Derivative Maturity Over Four Years From Balance Sheet Date [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount $ 7,735,000 $ 3,910,000
Derivative, Average Fixed Interest Rate 0.975% 0.96%
Derivative, Average Variable Interest Rate 0.294% 0.313%
Derivative, Weighted Average Remaining Maturity 4.811 4.723