Annual report pursuant to Section 13 and 15(d)

Derivative Instruments and Hedging Activities Schedule of Interest Rate Swaptions (Details)

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Derivative Instruments and Hedging Activities Schedule of Interest Rate Swaptions (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Options Held [Member]
    
Derivative [Line Items]    
Derivative, Cost $ 133,908 $ 29,670
Derivative Asset, Fair Value, Net 102,048 5,635
Derivative, Weighted Average Remaining Maturity 53.3756 12.2647
Underlying Swap [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 4,950,000 2,900,000
Derivative, Average Fixed Interest Rate 3.7525% 3.2065%
Derivative, Description of Variable Rate Basis 3M Libor 3M Libor
Derivative, Weighted Average Remaining Maturity 9.7576 4.931
Less Than Six Months Remaining Maturity [Member] | Options Held [Member]
    
Derivative [Line Items]    
Derivative, Lower Remaining Maturity Range < 6 Months < 6 Months
Derivative, Cost 3,983 16,147
Derivative Asset, Fair Value, Net 30 4
Derivative, Weighted Average Remaining Maturity 5.3745 4.9724
Less Than Six Months Remaining Maturity [Member] | Underlying Swap [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 300,000 1,600,000
Derivative, Average Fixed Interest Rate 4.00% 3.218%
Derivative, Description of Variable Rate Basis 3M Libor 3M Libor
Derivative, Weighted Average Remaining Maturity 10 3.688
Six Months or Longer Remaining Maturity [Member] | Options Held [Member]
    
Derivative [Line Items]    
Derivative, Higher Remaining Maturity Range ≥ 6 Months ≥ 6 Months
Derivative, Cost 129,925 13,523
Derivative Asset, Fair Value, Net 102,018 5,631
Derivative, Weighted Average Remaining Maturity 53.3834 12.2706
Six Months or Longer Remaining Maturity [Member] | Underlying Swap [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount $ 4,650,000 $ 1,300,000
Derivative, Average Fixed Interest Rate 3.7366% 3.1923%
Derivative, Description of Variable Rate Basis 3M Libor 3M Libor
Derivative, Weighted Average Remaining Maturity 9.742 6.462