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
 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Dec. 31, 2012
Options Held [Member]
      
Derivative [Line Items]      
Derivative, Cost $ 161,923   $ 133,908
Derivative Asset, Fair Value, Net 225,810   102,048
Derivative, Weighted Average Remaining Maturity 46.646   53.376
Underlying Swap [Member]
      
Derivative [Line Items]      
Derivative, Notional Amount 6,250,000   4,950,000
Derivative, Average Fixed Interest Rate 3.582%   3.753%
Derivative, Description of Variable Rate Basis 3M Libor 3M Libor  
Derivative, Weighted Average Remaining Maturity 9.208   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 28,213   3,983
Derivative Asset, Fair Value, Net 4,639   30
Derivative, Weighted Average Remaining Maturity 2.154   5.375
Less Than Six Months Remaining Maturity [Member] | Underlying Swap [Member]
      
Derivative [Line Items]      
Derivative, Notional Amount 2,750,000   300,000
Derivative, Average Fixed Interest Rate 3.127%   4.00%
Derivative, Description of Variable Rate Basis 3M Libor 3M Libor  
Derivative, Weighted Average Remaining Maturity 8.2   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 221,171   102,018
Derivative, Weighted Average Remaining Maturity 49.351   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