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Investments (Aggregated Income Statement Summarized Financial Information for Partnerhips in our Alternative Investment Portfolio) (Details) (USD $)
 3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Mar. 31, 2013
Alternative Investments [Member]
Mar. 31, 2012
Alternative Investments [Member]
Dec. 31, 2012
Partnerships accounted for under the equity method [Member]
Dec. 31, 2011
Partnerships accounted for under the equity method [Member]
Schedule of Equity Method Investments [Line Items]            
Net investment income         $ 204,800,000 $ 36,100,000
Realized gains         593,400,000 750,700,000
Net change in unrealized depreciation         (417,500,000) (487,400,000)
Net income         380,700,000 299,400,000
Net investment income earned $ 32,870,000 $ 32,628,000 $ 3,600,000 $ 2,000,000