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Investments (Details) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2012 | Dec. 31, 2011
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Gross unrealized gains and losses and the cost or amortized cost and fair value of investments classified as held-to-maturity |  |
Cost or Amortized Cost/Fixed-maturity securities-held to maturity, at amortized cost | $ 583,287 | $ 646,048
Gross Unrealized Gains | 44,361 | 40,573
Gross Unrealized Losses | (299) | (1,379)
Fair Value | 627,349 | 685,242
States and political subdivisions [Member] |  |
Gross unrealized gains and losses and the cost or amortized cost and fair value of investments classified as held-to-maturity |  |
Cost or Amortized Cost/Fixed-maturity securities-held to maturity, at amortized cost | 400,687 | 441,273
Gross Unrealized Gains | 31,387 | 29,026
Gross Unrealized Losses | (106) | (360)
Fair Value | 431,968 | 469,939
Corporate bonds [Member] |  |
Gross unrealized gains and losses and the cost or amortized cost and fair value of investments classified as held-to-maturity |  |
Cost or Amortized Cost/Fixed-maturity securities-held to maturity, at amortized cost | 82,824 | 92,682
Gross Unrealized Gains | 1,565 | 1,539
Gross Unrealized Losses | (12) | (130)
Fair Value | 84,377 | 94,091
Commercial mortgage-backed securities [Member] |  |
Gross unrealized gains and losses and the cost or amortized cost and fair value of investments classified as held-to-maturity |  |
Cost or Amortized Cost/Fixed-maturity securities-held to maturity, at amortized cost | 51,529 | 51,550
Gross Unrealized Gains | 6,433 | 4,195
Fair Value | 57,962 | 55,745
U.S. agency-based mortgage-backed securities [Member] |  |
Gross unrealized gains and losses and the cost or amortized cost and fair value of investments classified as held-to-maturity |  |
Cost or Amortized Cost/Fixed-maturity securities-held to maturity, at amortized cost | 32,984 | 46,096
Gross Unrealized Gains | 3,063 | 4,162
Gross Unrealized Losses | (5) | (44)
Fair Value | 36,042 | 50,214
U.S. Treasury securities and obligations of U.S. Government agencies [Member] |  |
Gross unrealized gains and losses and the cost or amortized cost and fair value of investments classified as held-to-maturity |  |
Cost or Amortized Cost/Fixed-maturity securities-held to maturity, at amortized cost | 11,034 | 9,141
Gross Unrealized Gains | 1,721 | 1,634
Fair Value | 12,755 | 10,775
Asset-backed securities [Member] |  |
Gross unrealized gains and losses and the cost or amortized cost and fair value of investments classified as held-to-maturity |  |
Cost or Amortized Cost/Fixed-maturity securities-held to maturity, at amortized cost | 4,229 | 5,306
Gross Unrealized Gains | 192 | 17
Gross Unrealized Losses | (176) | (845)
Fair Value | $ 4,245 | $ 4,478

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