Investments (Schedule of Available for Sale Securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Fixed Maturity & Equity Securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	$ 6,192,036	$ 6,352,737
Gross unrealized gain	286,591	280,620
Gross unrealized loss	(26,183)	(22,989)
Fixed maturity securities - available for sale	6,452,444	6,610,368
Available for sale equity securities
Available-for-sale Equity Securities, Amortized Cost Basis	325,435	291,070
Gross unrealized gain	25,464	24,069
Gross unrealized loss	(16,136)	(18,787)
Available-for-sale Securities, Equity Securities	334,763	296,352
US Government and Government Agency Securities [Member]
Fixed Maturity & Equity Securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	73,540	70,279
Gross unrealized gain	983	827
Gross unrealized loss	(7)	(137)
Fixed maturity securities - available for sale	74,516	70,969
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	877,954	896,130
Gross unrealized gain	56,164	58,738
Gross unrealized loss	(55)	(160)
Fixed maturity securities - available for sale	934,063	954,708
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	2,253,237	2,246,707
Gross unrealized gain	143,514	143,291
Gross unrealized loss	(1,065)	(986)
Fixed maturity securities - available for sale	2,395,686	2,389,012
Corporate Securities [Member]
Fixed Maturity & Equity Securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	726,320	777,242
Gross unrealized gain	28,287	25,983
Gross unrealized loss	(2,584)	(4,656)
Fixed maturity securities - available for sale	752,023	798,569
Residential Mortgage Backed Securities [Member]
Fixed Maturity & Equity Securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	770,957	805,458
Gross unrealized gain	22,259	20,215
Gross unrealized loss	(1,806)	(3,979)
Fixed maturity securities - available for sale	791,410	821,694
Commercial Mortgage Backed Securities [Member]
Fixed Maturity & Equity Securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	567,340	593,956
Gross unrealized gain	26,032	19,707
Gross unrealized loss	(592)	(2,032)
Fixed maturity securities - available for sale	592,780	611,631
Asset-backed Securities [Member]
Fixed Maturity & Equity Securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	369,289	369,103
Gross unrealized gain	1,051	316
Gross unrealized loss	(1,038)	(2,592)
Fixed maturity securities - available for sale	369,302	366,827
Foreign Government Securities [Member]
Fixed Maturity & Equity Securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	124,957	119,479
Gross unrealized gain	471	767
Gross unrealized loss	(3,396)	(1,554)
Fixed maturity securities - available for sale	122,032	118,692
Foreign Corporate Securities [Member]
Fixed Maturity & Equity Securities Available for Sale
Available-for-sale Fixed Maturity Securities, Amortized Cost Basis	428,442	474,383
Gross unrealized gain	7,830	10,776
Gross unrealized loss	(15,640)	(6,893)
Fixed maturity securities - available for sale	$ 420,632	$ 478,266

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