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Investments (Net Investment Income) (Details) (USD $) | 3 Months Ended | 12 Months Ended
--- | --- | ---
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 | Dec. 31, 2011 | Dec. 31, 2010
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
Decrease In Net Investment Income Before Tax |  |  |  |  |  |  |  |  | 15,600,000 |  |
Alternative Investments [Member] |  |  |  |  |  |  |  |  |  |  |
Decrease In Net Investment Income Before Tax |  |  |  |  |  |  |  |  | 10,300,000 |  |
Fixed Maturity Securities [Member] |  |  |  |  |  |  |  |  |  |  |
Net investment income earned |  |  |  |  |  |  |  |  | 124,687,000 | 129,710,000 | 130,990,000
Decrease In Net Investment Income Before Tax |  |  |  |  |  |  |  |  | 5,000,000 |  |
Equity Securities [Member] |  |  |  |  |  |  |  |  |  |  |
Net investment income earned |  |  |  |  |  |  |  |  | 6,215,000 | 4,535,000 | 2,238,000
Short-term Investments [Member] |  |  |  |  |  |  |  |  |  |  |
Net investment income earned |  |  |  |  |  |  |  |  | 151,000 | 160,000 | 437,000
Other Investment Portfolio [Member] |  |  |  |  |  |  |  |  |  |  |
Net investment income earned |  |  |  |  |  |  |  |  | 8,996,000 | 20,539,000 | 20,313,000
Miscellaneous Investment Income [Member] |  |  |  |  |  |  |  |  |  |  |
Net investment income earned |  |  |  |  |  |  |  |  | 0 | 133,000 | 139,000
Investment Expenses [Member] |  |  |  |  |  |  |  |  |  |  |
Net investment income earned |  |  |  |  |  |  |  |  | (8,172,000) | (7,634,000) | (8,409,000)
Fixed Matuirty Securities Which Matured Or Were Sold [Member] |  |  |  |  |  |  |  |  |  |  |
Fixed Maturity Securities Amortized Cost | 658,300,000 |  |  |  | 481,900,000 |  |  |  | 658,300,000 | 481,900,000 |
Average Investment Portfolio Yield |  |  |  |  |  |  |  |  | 3.70% | 3.80% |
Fixed Maturity Securities Which Were Purchased [Member] |  |  |  |  |  |  |  |  |  |  |
Fixed Maturity Securities Cost | $ 892,600,000 |  |  |  | $ 490,800,000 |  |  |  | $ 892,600,000 | $ 490,800,000 |
Average Investment Portfolio Yield |  |  |  |  |  |  |  |  | 2.20% | 2.70% |

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- Definition This item represents debt securities, at cost, net of adjustments made for amortization and other-than-temporary impairments. + References No definition available. + Details
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- Definition This item represents the increase (decrease) in income earned from investments in securities and other investments including dividends, interest, and returns on other investments in the current reporting period as compared to the comparable prior year period. This item does not include realized gains (losses). + References No definition available. + Details
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- Definition The income earned from investments in securities and other investments including dividends, interest, and returns on other investments. This item does not include realized gains (losses). + References No definition available. + Details
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