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 12 Months Ended
Jun. 30, 2012
Dec. 31, 2011
Options Held [Member]
    
Derivative [Line Items]    
Derivative, Lower Remaining Maturity Range < 6 Months < 6 Months
Derivative, Higher Remaining Maturity Range ≥ 6 Months ≥ 6 Months
Derivative, Cost $ 70,340 $ 29,670
Derivative Asset, Fair Value, Net 38,228 5,635
Derivative, Weighted Average Remaining Maturity 48.3276 12.2647
Underlying Swap [Member]
    
Derivative [Line Items]    
Derivative, Weighted Average Remaining Maturity 8.619 4.931
Derivative, Notional Amount 4,200,000 2,900,000
Derivative, Average Fixed Interest Rate 3.5179% 3.2065%
Derivative, Description of Variable Rate Basis 3M Libor 3M Libor
Six Months or Longer Remaining Maturity [Member] | Options Held [Member]
    
Derivative [Line Items]    
Derivative, Cost 60,800 13,523
Derivative Asset, Fair Value, Net 38,225 5,631
Derivative, Weighted Average Remaining Maturity 48.3324 12.2706
Six Months or Longer Remaining Maturity [Member] | Underlying Swap [Member]
    
Derivative [Line Items]    
Derivative, Weighted Average Remaining Maturity 9.625 6.462
Derivative, Notional Amount 3,200,000 1,300,000
Derivative, Average Fixed Interest Rate 3.6953% 3.1923%
Derivative, Description of Variable Rate Basis 3M Libor 3M Libor
Less Than Six Months Remaining Maturity [Member] | Options Held [Member]
    
Derivative [Line Items]    
Derivative, Cost 9,540 16,147
Derivative Asset, Fair Value, Net 3 4
Derivative, Weighted Average Remaining Maturity 3.3842 4.9724
Less Than Six Months Remaining Maturity [Member] | Underlying Swap [Member]
    
Derivative [Line Items]    
Derivative, Weighted Average Remaining Maturity 5.4 3.688
Derivative, Notional Amount $ 1,000,000 $ 1,600,000
Derivative, Average Fixed Interest Rate 2.95% 3.218%
Derivative, Description of Variable Rate Basis 3M Libor 3M Libor