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, 2011
Dec. 31, 2010
Options Held [Member]
    
Derivative [Line Items]    
Derivative, Lower Remaining Maturity Range < 6 Months  
Derivative, Cost $ 29,670  
Derivative Asset, Fair Value, Net 5,635  
Derivative, Average Remaining Maturity 12.2647  
Derivative, Higher Remaining Maturity Range ≥ 6 Months ≥ 6 Months
Underlying Swap [Member]
    
Derivative [Line Items]    
Derivative, Average Remaining Maturity 4.931  
Derivative, Notional Amount 2,900,000  
Derivative, Average Fixed Interest Rate 3.2065%  
Derivative, Description of Variable Rate Basis 3M Libor  
Less Than Six Months Remaining Maturity [Member] | Options Held [Member]
    
Derivative [Line Items]    
Derivative, Cost 16,147  
Derivative Asset, Fair Value, Net 4  
Derivative, Average Remaining Maturity 4.9724  
Less Than Six Months Remaining Maturity [Member] | Underlying Swap [Member]
    
Derivative [Line Items]    
Derivative, Average Remaining Maturity 3.688  
Derivative, Notional Amount 1,600,000  
Derivative, Average Fixed Interest Rate 3.218%  
Derivative, Description of Variable Rate Basis 3M Libor  
Six Months or Longer Remaining Maturity [Member] | Options Held [Member]
    
Derivative [Line Items]    
Derivative, Cost 13,523 3,348
Derivative Asset, Fair Value, Net 5,631 4,028
Derivative, Average Remaining Maturity 12.2706 11.225
Six Months or Longer Remaining Maturity [Member] | Underlying Swap [Member]
    
Derivative [Line Items]    
Derivative, Average Remaining Maturity 6.462 8.5
Derivative, Notional Amount $ 1,300,000 $ 100,000
Derivative, Average Fixed Interest Rate 3.1923% 3.52%
Derivative, Description of Variable Rate Basis 3M Libor 3M Libor