Quarterly report pursuant to Section 13 or 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
 3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Dec. 31, 2012
Options Held [Member]
      
Derivative [Line Items]      
Derivative, Cost $ 162,680   $ 133,908
Derivative Asset, Fair Value, Net 148,791   102,048
Derivative, Weighted Average Remaining Maturity 52.942   53.376
Underlying Swap [Member]
      
Derivative [Line Items]      
Derivative, Notional Amount 5,800,000   4,950,000
Derivative, Average Fixed Interest Rate 3.873%   3.753%
Derivative, Description of Variable Rate Basis 3M Libor 3M Libor  
Derivative, Weighted Average Remaining Maturity 9.793   9.758
Less Than Six Months Remaining Maturity [Member] | Options Held [Member]
      
Derivative [Line Items]      
Derivative, Lower Remaining Maturity Range < 6 Months   < 6 Months
Derivative, Cost 28,970   3,983
Derivative Asset, Fair Value, Net 382   30
Derivative, Weighted Average Remaining Maturity 4.902   5.375
Less Than Six Months Remaining Maturity [Member] | Underlying Swap [Member]
      
Derivative [Line Items]      
Derivative, Notional Amount 2,300,000   300,000
Derivative, Average Fixed Interest Rate 3.772%   4.00%
Derivative, Description of Variable Rate Basis 3M Libor 3M Libor  
Derivative, Weighted Average Remaining Maturity 9.478   10
Six Months or Longer Remaining Maturity [Member] | Options Held [Member]
      
Derivative [Line Items]      
Derivative, Higher Remaining Maturity Range ≥ 6 Months   ≥ 6 Months
Derivative, Cost 133,710   129,925
Derivative Asset, Fair Value, Net 148,409   102,018
Derivative, Weighted Average Remaining Maturity 53.031   53.383
Six Months or Longer Remaining Maturity [Member] | Underlying Swap [Member]
      
Derivative [Line Items]      
Derivative, Notional Amount $ 3,500,000   $ 4,650,000
Derivative, Average Fixed Interest Rate 3.939%   3.737%
Derivative, Description of Variable Rate Basis 3M Libor 3M Libor  
Derivative, Weighted Average Remaining Maturity 10   9.742