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Investments (Other Investment Portfolio by Strategy and the Remaining Commitment Amount Associated With Each Strategy) (Details) (USD $) | 3 Months Ended | 9 Months Ended |  | 9 Months Ended |  | 3 Months Ended | 9 Months Ended |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 9 Months Ended
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Sep. 30, 2012 | Sep. 30, 2011 | Sep. 30, 2012 | Sep. 30, 2011 | Dec. 31, 2011 | Sep. 30, 2012 Other Investments [Member] | Dec. 31, 2011 Other Investments [Member] | Jun. 30, 2012 Alternative Investments [Member] | Jun. 30, 2011 Alternative Investments [Member] | Jun. 30, 2012 Alternative Investments [Member] | Jun. 30, 2011 Alternative Investments [Member] | Sep. 30, 2012 Alternative Investments [Member] | Dec. 31, 2011 Alternative Investments [Member] | Sep. 30, 2012 Secondary Private Equity Member Alternative Investments [Member] | Dec. 31, 2011 Secondary Private Equity Member Alternative Investments [Member] | Sep. 30, 2012 Private Equity [Member] Alternative Investments [Member] | Dec. 31, 2011 Private Equity [Member] Alternative Investments [Member] | Sep. 30, 2012 Energy Power Generation [Member] Alternative Investments [Member] | Dec. 31, 2011 Energy Power Generation [Member] Alternative Investments [Member] | Sep. 30, 2012 Distressed Debt [Member] Alternative Investments [Member] | Dec. 31, 2011 Distressed Debt [Member] Alternative Investments [Member] | Sep. 30, 2012 Real Estate Strategy [Member] Alternative Investments [Member] | Dec. 31, 2011 Real Estate Strategy [Member] Alternative Investments [Member] | Sep. 30, 2012 Mezzanine Financing [Member] Alternative Investments [Member] | Dec. 31, 2011 Mezzanine Financing [Member] Alternative Investments [Member] | Sep. 30, 2012 Venture Capital [Member] Alternative Investments [Member] | Dec. 31, 2011 Venture Capital [Member] Alternative Investments [Member] | Sep. 30, 2012 Other Securities [Member] | Dec. 31, 2011 Other Securities [Member] | Sep. 30, 2012 Previously Existing Partnership Commitments [Member] | Sep. 30, 2012 New Mezzanine Limited Partnerships [Member]
Increase Decrease In Net Investment Income Before Tax | $ (5,100,000) | $ (21,300,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Carrying Value | 122,081,000 |  | 122,081,000 |  | 128,301,000 | 122,081,000 | 128,301,000 |  |  |  |  | 117,784,000 | 124,548,000 | 28,883,000 | 30,114,000 | 24,743,000 | 21,736,000 | 19,198,000 | 25,913,000 | 13,413,000 | 16,953,000 | 12,719,000 | 13,767,000 | 11,177,000 | 8,817,000 | 7,651,000 | 7,248,000 | 4,297,000 | 3,753,000 |  |
Remaining Commitment |  |  |  |  |  | 64,846,000 |  |  |  |  |  | 63,787,000 |  | 8,153,000 |  | 3,883,000 |  | 10,347,000 |  | 7,357,000 |  | 10,473,000 |  | 23,174,000 |  | 400,000 |  | 1,059,000 |  |  | 7,900,000
Decrease in other investments |  |  | (6,200,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Distributions from alternative investments |  |  | 19,700,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net investment income earned | 30,650,000 | 35,786,000 | 97,284,000 | 118,604,000 |  | 5,200,000 |  | 500,000 | 4,500,000 | 5,500,000 | 24,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional contributions |  |  | $ 8,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 5,400,000 | $ 2,500,000

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