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Invesments (Aggregated Income Statement Summarized Financial Information for Partnerhips in our Alternative Investment Portfolio) (Details) (USD $)
 3 Months Ended 12 Months Ended
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2010
Sep. 30, 2010
Jun. 30, 2010
Mar. 31, 2010
Dec. 31, 2011
Sep. 30, 2011
Dec. 31, 2010
Sep. 30, 2010
Dec. 31, 2009
Sep. 30, 2009
Net investment income                   $ 564,000,000   $ 563,000,000   $ 472,000,000
Realized gains (losses)                   893,000,000   (358,000,000)   (499,000,000)
Net change in unrealized appreciation                   1,485,000,000   2,250,000,000   (3,063,000,000)
Net income                   2,942,000,000   2,455,000,000   (3,090,000,000)
Net investment income earned 28,839,000 35,786,000 39,345,000 43,473,000 41,471,000 32,986,000 36,545,000 34,706,000 147,443,000   145,708,000   118,471,000  
Alternative Investments [Member]
                            
Net investment income earned                 $ 21,000,000   $ 20,000,000   $ (22,000,000)