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Invesments (Aggregated Income Statement Summarized Financial Information for Partnerhips in our Alternative Investment Portfolio) (Details) (USD $)
 3 Months Ended
Mar. 31, 2012
Dec. 31, 2011
Mar. 31, 2011
Dec. 31, 2010
Schedule of Equity Method Investments [Line Items]        
Net investment income   $ 36,100,000   $ 154,200,000
Realized gains (losses)   750,700,000   (192,300,000)
Net change in unrealized (depreciation) appreciation   (487,400,000)   1,464,200,000
Net income   299,400,000   1,426,100,000
Net investment income earned 32,628,000   43,473,000  
Alternative Investments [Member]
        
Schedule of Equity Method Investments [Line Items]        
Net investment income earned $ 2,000,000   $ 11,600,000