Investments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
U.S. Treasury obligations [Member]
Fixed maturities, available for sale
Amortized Cost	$ 163,714	$ 166,115
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	3,785	6,118
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	987	1,519
Total Fair Value	166,512	170,714
U.S. Government-sponsored enterprise obligations [Member]
Fixed maturities, available for sale
Amortized Cost	38,022	30,942
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1,641	2,251
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	100	425
Total Fair Value	39,563	32,768
State and municipal bonds [Member]
Fixed maturities, available for sale
Amortized Cost	1,015,555	1,116,060
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	47,395	46,533
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	335	7,927
Total Fair Value	1,062,615	1,154,666
Corporate debt [Member]
Fixed maturities, available for sale
Amortized Cost	1,389,970	1,321,838
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	44,234	53,059
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	17,103	13,744
Total Fair Value	1,417,101	1,361,153
Residential mortgage-backed securities [Member]
Fixed maturities, available for sale
Amortized Cost	266,306	230,861
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	10,198	7,608
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	448	2,855
Total Fair Value	276,056	235,614
Agency commercial mortgage-backed securities [Member]
Fixed maturities, available for sale
Amortized Cost	15,344	27,268
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	208	343
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	59	136
Total Fair Value	15,493	27,475
Other commercial mortgage-backed securities [Member]
Fixed maturities, available for sale
Amortized Cost	50,025	59,066
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1,137	2,491
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	99	167
Total Fair Value	51,063	61,390
Other asset-backed securities [Member]
Fixed maturities, available for sale
Amortized Cost	116,541	74,106
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	288	487
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	205	324
Total Fair Value	116,624	74,269
Debt Securities [Member]
Fixed maturities, available for sale
Amortized Cost	3,055,477	3,026,256
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	108,886	118,890
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	19,336	27,097
Total Fair Value	$ 3,145,027	$ 3,118,049

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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