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Investments (Details) (USD $)
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Jun. 30, 2013
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Dec. 31, 2012
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U.S. Treasury obligations [Member]
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| Fixed maturities, available for sale | ||||||
| Amortized Cost | $ 206,778,000 | $ 191,642,000 | ||||
| Gross Unrealized Gains | 8,427,000 | 14,266,000 | ||||
| Gross Unrealized Losses | (1,383,000) | (51,000) | ||||
| Total Fair Value | 213,822,000 | 205,857,000 | ||||
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U.S. Government-sponsored enterprise obligations [Member]
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| Fixed maturities, available for sale | ||||||
| Amortized Cost | 53,486,000 | 52,110,000 | ||||
| Gross Unrealized Gains | 3,732,000 | 4,837,000 | ||||
| Gross Unrealized Losses | (293,000) | 0 | ||||
| Total Fair Value | 56,925,000 | 56,947,000 | ||||
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State and municipal bonds [Member]
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| Fixed maturities, available for sale | ||||||
| Amortized Cost | 1,204,739,000 | 1,134,744,000 | ||||
| Gross Unrealized Gains | 55,546,000 | 85,329,000 | ||||
| Gross Unrealized Losses | (7,113,000) | (94,000) | ||||
| Total Fair Value | 1,253,172,000 | 1,219,979,000 | ||||
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Corporate debt [Member]
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| Fixed maturities, available for sale | ||||||
| Amortized Cost | 1,502,700,000 | 1,375,880,000 | ||||
| Gross Unrealized Gains | 60,506,000 | 96,187,000 | ||||
| Gross Unrealized Losses | (16,314,000) | (1,543,000) | ||||
| Total Fair Value | 1,546,892,000 | 1,470,524,000 | ||||
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Residential mortgage-backed securities [Member]
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| Schedule of Available-for-sale Securities [Line Items] | ||||||
| Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities | 500,000 | 900,000 | ||||
| Fixed maturities, available for sale | ||||||
| Amortized Cost | 263,844,000 | 272,990,000 | ||||
| Gross Unrealized Gains | 10,382,000 | 17,070,000 | ||||
| Gross Unrealized Losses | (1,993,000) | [1] | (210,000) | [1] | ||
| Total Fair Value | 272,233,000 | 289,850,000 | ||||
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Agency commercial mortgage-backed securities [Member]
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| Fixed maturities, available for sale | ||||||
| Amortized Cost | 43,696,000 | 57,234,000 | ||||
| Gross Unrealized Gains | 822,000 | 2,255,000 | ||||
| Gross Unrealized Losses | (69,000) | (25,000) | ||||
| Total Fair Value | 44,449,000 | 59,464,000 | ||||
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Other commercial mortgage-backed securities [Member]
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| Fixed maturities, available for sale | ||||||
| Amortized Cost | 67,861,000 | 69,062,000 | ||||
| Gross Unrealized Gains | 3,264,000 | 5,049,000 | ||||
| Gross Unrealized Losses | (260,000) | (5,000) | ||||
| Total Fair Value | 70,865,000 | 74,106,000 | ||||
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Other asset-backed securities [Member]
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| Fixed maturities, available for sale | ||||||
| Amortized Cost | 75,843,000 | 70,670,000 | ||||
| Gross Unrealized Gains | 625,000 | 1,203,000 | ||||
| Gross Unrealized Losses | (393,000) | (601,000) | ||||
| Total Fair Value | 76,075,000 | 71,272,000 | ||||
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Debt Securities [Member]
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| Fixed maturities, available for sale | ||||||
| Amortized Cost | 3,418,947,000 | 3,224,332,000 | ||||
| Gross Unrealized Gains | 143,304,000 | 226,196,000 | ||||
| Gross Unrealized Losses | (27,818,000) | (2,529,000) | ||||
| Total Fair Value | $ 3,534,433,000 | $ 3,447,999,000 | ||||
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