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Investments (Details 5) (USD $)
Sep. 30, 2012
Dec. 31, 2011
Corporate Secturities by Investment Sector And Credit Rating [Line Items]    
Available For Sale Securities Amortized Cost $ 10,885,071,000 $ 6,931,958,000
Available For Sale Securities Fair Value Disclosure 11,899,722,000 7,461,106,000
Percentage Of Total Available For Sale Securities 100.00% 100.00%
Finance Industry [Member]
   
Corporate Secturities by Investment Sector And Credit Rating [Line Items]    
Available For Sale Securities Amortized Cost 3,551,382,000 2,411,175,000
Available For Sale Securities Fair Value Disclosure 3,802,951,000 2,442,149,000
Percentage Of Total Available For Sale Securities 32.00% 32.70%
Industrial [Member]
   
Corporate Secturities by Investment Sector And Credit Rating [Line Items]    
Available For Sale Securities Amortized Cost 5,583,541,000 3,402,099,000
Available For Sale Securities Fair Value Disclosure 6,146,069,000 3,760,187,000
Percentage Of Total Available For Sale Securities 51.60% 50.40%
Utility Industry [Member]
   
Corporate Secturities by Investment Sector And Credit Rating [Line Items]    
Available For Sale Securities Amortized Cost 1,715,512,000 1,115,384,000
Available For Sale Securities Fair Value Disclosure 1,914,809,000 1,255,090,000
Percentage Of Total Available For Sale Securities 16.10% 16.90%
Other Industry [Member]
   
Corporate Secturities by Investment Sector And Credit Rating [Line Items]    
Available For Sale Securities Amortized Cost 34,636,000 3,300,000
Available For Sale Securities Fair Value Disclosure $ 35,893,000 $ 3,680,000
Percentage Of Total Available For Sale Securities 0.30% 0.00%