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Investments (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, 2012 | Sep. 30, 2012 | Jun. 30, 2012 | Mar. 31, 2012 | Dec. 31, 2011 | Sep. 30, 2011 | Jun. 30, 2011 | Mar. 31, 2011 | Dec. 31, 2012 | Sep. 30, 2012 | Dec. 31, 2011 | Sep. 30, 2011 | Dec. 31, 2010 | Sep. 30, 2010
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Investment Income |  |  |  |  |  |  |  |  |  | $ 226,000,000 |  | $ 564,000,000 |  | $ 563,000,000
Realized gains (losses) |  |  |  |  |  |  |  |  |  | 1,015,000,000 |  | 893,000,000 |  | (358,000,000)
Net change in unrealized appreciation (depreciation) |  |  |  |  |  |  |  |  |  | (100,000,000) |  | 1,485,000,000 |  | 2,250,000,000
Net Income |  |  |  |  |  |  |  |  |  | 1,141,000,000 |  | 2,942,000,000 |  | 2,455,000,000
Net investment income earned | 34,593,000 | 30,650,000 | 34,006,000 | 32,628,000 | 28,839,000 | 35,786,000 | 39,345,000 | 43,473,000 | 131,877,000 |  | 147,443,000 |  | 145,708,000 |
Partnership Interest [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net investment income earned |  |  |  |  |  |  |  |  | $ 9,000,000 |  | $ 21,000,000 |  | $ 20,000,000 |

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