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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
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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) |

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