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Investments (Other Investment Portfolio by Strategy and the Remaining Commitment Amount Associated With Each Strategy) (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 | Dec. 31, 2010 | Dec. 31, 2009
Carrying Value | $ 128,301,000 |  |  |  | $ 137,865,000 |  |  |  | $ 128,301,000 | $ 137,865,000 |
Decrease in other investments |  |  |  |  |  |  |  |  | (9,600,000) |  |
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
Other Investments [Member] |  |  |  |  |  |  |  |  |  |  |
Carrying Value | 128,301,000 |  |  |  | 137,865,000 |  |  |  | 128,301,000 | 137,865,000 |
Remaining Commitment | 56,865,000 |  |  |  |  |  |  |  | 56,865,000 |  |
Distributions from alternative investments |  |  |  |  |  |  |  |  | 45,800,000 |  |
Net investment income earned |  |  |  |  |  |  |  |  | 20,500,000 |  |
Additional contributions |  |  |  |  |  |  |  |  | 16,100,000 |  |
Alternative Investments [Member] |  |  |  |  |  |  |  |  |  |  |
Carrying Value | 124,548,000 |  |  |  | 135,110,000 |  |  |  | 124,548,000 | 135,110,000 |
Remaining Commitment | 55,344,000 |  |  |  |  |  |  |  | 55,344,000 |  |
Net investment income earned |  |  |  |  |  |  |  |  | 21,000,000 | 20,000,000 | (22,000,000)
Secondary Private Equity Member | Alternative Investments [Member] |  |  |  |  |  |  |  |  |  |  |
Carrying Value | 30,114,000 |  |  |  | 26,709,000 |  |  |  | 30,114,000 | 26,709,000 |
Remaining Commitment | 9,507,000 |  |  |  |  |  |  |  | 9,507,000 |  |
Energy/Power Generation [Member] | Alternative Investments [Member] |  |  |  |  |  |  |  |  |  |  |
Carrying Value | 25,913,000 |  |  |  | 35,560,000 |  |  |  | 25,913,000 | 35,560,000 |
Remaining Commitment | 10,608,000 |  |  |  |  |  |  |  | 10,608,000 |  |
Private Equity [Member] | Alternative Investments [Member] |  |  |  |  |  |  |  |  |  |  |
Carrying Value | 21,736,000 |  |  |  | 21,601,000 |  |  |  | 21,736,000 | 21,601,000 |
Remaining Commitment | 5,368,000 |  |  |  |  |  |  |  | 5,368,000 |  |
Distressed Debt [Member] | Alternative Investments [Member] |  |  |  |  |  |  |  |  |  |  |
Carrying Value | 16,953,000 |  |  |  | 20,432,000 |  |  |  | 16,953,000 | 20,432,000 |
Remaining Commitment | 3,136,000 |  |  |  |  |  |  |  | 3,136,000 |  |
Real Estate Strategy [Member] | Alternative Investments [Member] |  |  |  |  |  |  |  |  |  |  |
Carrying Value | 13,767,000 |  |  |  | 14,192,000 |  |  |  | 13,767,000 | 14,192,000 |
Remaining Commitment | 10,569,000 |  |  |  |  |  |  |  | 10,569,000 |  |
Mezzanine Financing [Member] | Alternative Investments [Member] |  |  |  |  |  |  |  |  |  |  |
Carrying Value | 8,817,000 |  |  |  | 10,230,000 |  |  |  | 8,817,000 | 10,230,000 |
Remaining Commitment | 15,256,000 |  |  |  |  |  |  |  | 15,256,000 |  |
Venture Capital [Member] | Alternative Investments [Member] |  |  |  |  |  |  |  |  |  |  |
Carrying Value | 7,248,000 |  |  |  | 6,386,000 |  |  |  | 7,248,000 | 6,386,000 |
Remaining Commitment | 900,000 |  |  |  |  |  |  |  | 900,000 |  |
Other Securities [Member] |  |  |  |  |  |  |  |  |  |  |
Carrying Value | 3,753,000 |  |  |  | 2,755,000 |  |  |  | 3,753,000 | 2,755,000 |
Remaining Commitment | $ 1,521,000 |  |  |  |  |  |  |  | $ 1,521,000 |  |

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