Summary of Investments (Fair Value of Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Held-to-maturity Securities, Debt Maturities [Abstract]
Cost or Amortized Cost	$ 356	$ 397
Fair Value	359	404
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	2,843,014	2,845,217
Gross Unrealized Appreciation	236,085	257,633
Gross Unrealized Depreciation	23,191	13,928
Fair Value	3,055,908	3,088,922
Fixed Maturities [Member]
Held-to-maturity Securities, Debt Maturities [Abstract]
Cost or Amortized Cost	356	397
Gross Unrealized Appreciation	3	7
Gross Unrealized Depreciation	0	0
Fair Value	359	404
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	2,770,857	2,773,566
Gross Unrealized Appreciation	63,349	82,882
Gross Unrealized Depreciation	22,845	13,369
Fair Value	2,811,361	2,843,079
States, Municipalities and Political Subdivisions [Member] | General Obligations [Member] | Midwest [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	161,011	179,491
Gross Unrealized Appreciation	4,801	6,599
Gross Unrealized Depreciation	684	170
Fair Value	165,128	185,920
States, Municipalities and Political Subdivisions [Member] | General Obligations [Member] | Northeast [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	55,662	59,084
Gross Unrealized Appreciation	1,276	2,120
Gross Unrealized Depreciation	293	50
Fair Value	56,645	61,154
States, Municipalities and Political Subdivisions [Member] | General Obligations [Member] | South [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	126,404	122,055
Gross Unrealized Appreciation	2,988	4,453
Gross Unrealized Depreciation	1,077	288
Fair Value	128,315	126,220
States, Municipalities and Political Subdivisions [Member] | General Obligations [Member] | West [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	95,579	75,102
Gross Unrealized Appreciation	2,363	3,350
Gross Unrealized Depreciation	789	19
Fair Value	97,153	78,433
States, Municipalities and Political Subdivisions [Member] | Special Revenue [Member] | Midwest [Member]
Held-to-maturity Securities, Debt Maturities [Abstract]
Cost or Amortized Cost	55	55
Gross Unrealized Appreciation	0	0
Gross Unrealized Depreciation	0	0
Fair Value	55	55
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	145,486	126,192
Gross Unrealized Appreciation	4,239	5,356
Gross Unrealized Depreciation	911	146
Fair Value	148,814	131,402
States, Municipalities and Political Subdivisions [Member] | Special Revenue [Member] | Northeast [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	19,515	11,767
Gross Unrealized Appreciation	633	864
Gross Unrealized Depreciation	242	0
Fair Value	19,906	12,631
States, Municipalities and Political Subdivisions [Member] | Special Revenue [Member] | South [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	103,185	106,917
Gross Unrealized Appreciation	3,423	4,368
Gross Unrealized Depreciation	556	63
Fair Value	106,052	111,222
States, Municipalities and Political Subdivisions [Member] | Special Revenue [Member] | West [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	77,401	68,024
Gross Unrealized Appreciation	2,518	3,285
Gross Unrealized Depreciation	388	6
Fair Value	79,531	71,303
Corporate Bonds [Member] | Energy [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	113,766	135,068
Gross Unrealized Appreciation	2,545	2,858
Gross Unrealized Depreciation	721	793
Fair Value	115,590	137,133
Corporate Bonds [Member] | Industrials [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	220,379	211,256
Gross Unrealized Appreciation	5,247	6,373
Gross Unrealized Depreciation	3,022	2,154
Fair Value	222,604	215,475
Corporate Bonds [Member] | Consumer Goods and Services [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	180,165	172,623
Gross Unrealized Appreciation	4,007	4,702
Gross Unrealized Depreciation	664	324
Fair Value	183,508	177,001
Corporate Bonds [Member] | Health Care [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	93,333	86,017
Gross Unrealized Appreciation	2,602	3,228
Gross Unrealized Depreciation	964	210
Fair Value	94,971	89,035
Corporate Bonds [Member] | Technology, Media and Telecommunications [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	144,328	131,465
Gross Unrealized Appreciation	2,345	3,863
Gross Unrealized Depreciation	1,339	799
Fair Value	145,334	134,529
Corporate Bonds [Member] | Financial Services [Member]
Held-to-maturity Securities, Debt Maturities [Abstract]
Cost or Amortized Cost	200	200
Gross Unrealized Appreciation	0	0
Gross Unrealized Depreciation	0	0
Fair Value	200	200
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	255,165	215,095
Gross Unrealized Appreciation	7,071	8,574
Gross Unrealized Depreciation	1,069	87
Fair Value	261,167	223,582
Mortgage-Backed Securities [Member]
Held-to-maturity Securities, Debt Maturities [Abstract]
Cost or Amortized Cost	101	142
Gross Unrealized Appreciation	3	7
Gross Unrealized Depreciation	0	0
Fair Value	104	149
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	15,096	17,121
Gross Unrealized Appreciation	485	483
Gross Unrealized Depreciation	60	46
Fair Value	15,521	17,558
U.S. Treasury [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	20,723	25,856
Gross Unrealized Appreciation	199	168
Gross Unrealized Depreciation	7	52
Fair Value	20,915	25,972
U.S. Government Agency [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	268,120	349,747
Gross Unrealized Appreciation	2,468	4,347
Gross Unrealized Depreciation	3,275	2,422
Fair Value	267,313	351,672
Foreign Bonds [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	101,857	136,487
Gross Unrealized Appreciation	3,504	4,132
Gross Unrealized Depreciation	167	446
Fair Value	105,194	140,173
Public Utilities [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	199,769	206,366
Gross Unrealized Appreciation	4,903	6,479
Gross Unrealized Depreciation	936	488
Fair Value	203,736	212,357
Collateralized Mortgage Obligations [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	368,182	335,092
Gross Unrealized Appreciation	5,493	7,003
Gross Unrealized Depreciation	5,667	4,806
Fair Value	368,008	337,289
Asset-Backed Securities [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	5,731	2,741
Gross Unrealized Appreciation	239	277
Gross Unrealized Depreciation	14	0
Fair Value	5,956	3,018
Equity Securities [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	72,157	71,651
Gross Unrealized Appreciation	172,736	174,751
Gross Unrealized Depreciation	346	559
Fair Value	244,547	245,843
Common Stocks [Member] | Public Utilities [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	7,231	7,231
Gross Unrealized Appreciation	10,365	13,103
Gross Unrealized Depreciation	84	44
Fair Value	17,512	20,290
Common Stocks [Member] | Energy [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	5,625	5,094
Gross Unrealized Appreciation	7,494	8,623
Gross Unrealized Depreciation	15	0
Fair Value	13,104	13,717
Common Stocks [Member] | Industrials [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	13,252	13,284
Gross Unrealized Appreciation	33,571	32,299
Gross Unrealized Depreciation	153	124
Fair Value	46,670	45,459
Common Stocks [Member] | Consumer Goods and Services [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	10,308	10,294
Gross Unrealized Appreciation	12,445	13,295
Gross Unrealized Depreciation	3	275
Fair Value	22,750	23,314
Common Stocks [Member] | Health Care [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	7,920	7,920
Gross Unrealized Appreciation	24,433	22,436
Gross Unrealized Depreciation	0	0
Fair Value	32,353	30,356
Common Stocks [Member] | Technology, Media and Telecommunications [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	6,151	6,207
Gross Unrealized Appreciation	7,086	7,846
Gross Unrealized Depreciation	30	58
Fair Value	13,207	13,995
Common Stocks [Member] | Financial Services [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	17,302	16,637
Gross Unrealized Appreciation	77,254	77,077
Gross Unrealized Depreciation	61	51
Fair Value	94,495	93,663
Nonredeemable Preferred Stocks [Member]
Available-for-sale Securities [Abstract]
Cost or Amortized Cost	4,368	4,984
Gross Unrealized Appreciation	88	72
Gross Unrealized Depreciation	0	7
Fair Value	$ 4,456	$ 5,049