Investments (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Fixed Maturity & Equity Securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	$ 6,352,737	$ 5,921,487
Gross unrealized gain	280,620	200,443
Gross unrealized loss	(22,989)	(99,457)
Fixed maturity securities - available for sale	6,610,368	6,022,473
Available for sale equity securities
Available-for-sale Equity Securities, Amortized Cost Basis	291,070	464,388
Gross unrealized gain	24,069	58,842
Gross unrealized loss	(18,787)	(5,764)
Available-for-sale Securities, Equity Securities	296,352	517,466
US Government and Government Agency Securities [Member]
Fixed Maturity & Equity Securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	70,279	91,047
Gross unrealized gain	827	2,157
Gross unrealized loss	(137)	(495)
Fixed maturity securities - available for sale	70,969	92,709
Fixed Maturity Securities of States, Municipalities and Political Subdivisions Securities [Member]
Fixed Maturity & Equity Securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	896,130	941,580
Gross unrealized gain	58,738	50,885
Gross unrealized loss	(160)	(5,979)
Fixed maturity securities - available for sale	954,708	986,486
Special Purpose Revenue Bonds Of States Municipalities And Political Subdivisions [Member]
Fixed Maturity & Equity Securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	2,246,707	2,240,412
Gross unrealized gain	143,291	71,541
Gross unrealized loss	(986)	(46,758)
Fixed maturity securities - available for sale	2,389,012	2,265,195
Corporate Securities [Member]
Fixed Maturity & Equity Securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	1,251,625	1,195,387
Gross unrealized gain	36,759	40,860
Gross unrealized loss	(11,549)	(11,009)
Fixed maturity securities - available for sale	1,276,835	1,225,238
Residential Mortgage Backed Securities [Member]
Fixed Maturity & Equity Securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	805,458	622,766
Gross unrealized gain	20,215	15,289
Gross unrealized loss	(3,979)	(19,936)
Fixed maturity securities - available for sale	821,694	618,119
Commercial Mortgage Backed Securities [Member]
Fixed Maturity & Equity Securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	593,956	502,069
Gross unrealized gain	19,707	16,155
Gross unrealized loss	(2,032)	(13,336)
Fixed maturity securities - available for sale	611,631	504,888
Asset-backed Securities [Member]
Fixed Maturity & Equity Securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	369,103	183,660
Gross unrealized gain	316	319
Gross unrealized loss	(2,592)	(1,587)
Fixed maturity securities - available for sale	366,827	182,392
Foreign Government Securities [Member]
Fixed Maturity & Equity Securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	119,479	144,566
Gross unrealized gain	767	3,237
Gross unrealized loss	(1,554)	(357)
Fixed maturity securities - available for sale	$ 118,692	$ 147,446

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