Summary of Investments (Fair Value of Investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
HELD-TO-MATURITY
Cost or Amortized Cost	$ 664	$ 672
Fair Value	667	675
AVAILABLE-FOR-SALE
Cost or Amortized Cost	2,839,442	2,861,583
Gross Unrealized Appreciation	266,074	227,716
Gross Unrealized Depreciation	12,534	28,091
Fair Value	3,092,982	3,061,208
Fixed Maturities [Member]
HELD-TO-MATURITY
Cost or Amortized Cost	664	672
Gross Unrealized Appreciation	3	3
Gross Unrealized Depreciation	0	0
Fair Value	667	675
AVAILABLE-FOR-SALE
Cost or Amortized Cost	2,770,718	2,793,069
Gross Unrealized Appreciation	96,256	58,994
Gross Unrealized Depreciation	11,808	27,102
Fair Value	2,855,166	2,824,961
States, Municipalities and Political Subdivisions [Member] | General Obligations [Member] | Midwest [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	160,686	160,484
Gross Unrealized Appreciation	5,464	4,990
Gross Unrealized Depreciation	14	18
Fair Value	166,136	165,456
States, Municipalities and Political Subdivisions [Member] | General Obligations [Member] | Northeast [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	61,333	56,449
Gross Unrealized Appreciation	2,594	1,996
Gross Unrealized Depreciation	0	0
Fair Value	63,927	58,445
States, Municipalities and Political Subdivisions [Member] | General Obligations [Member] | South [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	119,448	125,565
Gross Unrealized Appreciation	3,971	3,358
Gross Unrealized Depreciation	0	134
Fair Value	123,419	128,789
States, Municipalities and Political Subdivisions [Member] | General Obligations [Member] | West [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	106,544	103,721
Gross Unrealized Appreciation	3,949	3,160
Gross Unrealized Depreciation	49	67
Fair Value	110,444	106,814
States, Municipalities and Political Subdivisions [Member] | Special Revenue [Member] | Midwest [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	156,902	152,780
Gross Unrealized Appreciation	6,186	4,956
Gross Unrealized Depreciation	0	30
Fair Value	163,088	157,706
States, Municipalities and Political Subdivisions [Member] | Special Revenue [Member] | Northeast [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	29,203	23,892
Gross Unrealized Appreciation	1,143	919
Gross Unrealized Depreciation	4	212
Fair Value	30,342	24,599
States, Municipalities and Political Subdivisions [Member] | Special Revenue [Member] | South [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	160,498	144,183
Gross Unrealized Appreciation	5,779	4,281
Gross Unrealized Depreciation	7	27
Fair Value	166,270	148,437
States, Municipalities and Political Subdivisions [Member] | Special Revenue [Member] | West [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	80,862	78,935
Gross Unrealized Appreciation	3,796	3,150
Gross Unrealized Depreciation	2	44
Fair Value	84,656	82,041
Corporate Bonds [Member] | Energy [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	116,341	116,800
Gross Unrealized Appreciation	1,763	1,032
Gross Unrealized Depreciation	3,422	4,713
Fair Value	114,682	113,119
Corporate Bonds [Member] | Industrials [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	239,448	227,589
Gross Unrealized Appreciation	7,950	3,329
Gross Unrealized Depreciation	3,276	6,663
Fair Value	244,122	224,255
Corporate Bonds [Member] | Consumer Goods and Services [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	176,258	172,529
Gross Unrealized Appreciation	7,529	2,844
Gross Unrealized Depreciation	3	776
Fair Value	183,784	174,597
Corporate Bonds [Member] | Health Care [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	87,699	92,132
Gross Unrealized Appreciation	3,870	2,168
Gross Unrealized Depreciation	43	791
Fair Value	91,526	93,509
Corporate Bonds [Member] | Technology, Media and Telecommunications [Member]
HELD-TO-MATURITY
Cost or Amortized Cost	450	450
Gross Unrealized Appreciation	2	1
Gross Unrealized Depreciation	0	0
Fair Value	452	451
AVAILABLE-FOR-SALE
Cost or Amortized Cost	141,213	142,431
Gross Unrealized Appreciation	4,394	1,972
Gross Unrealized Depreciation	1,070	2,003
Fair Value	144,537	142,400
Corporate Bonds [Member] | Financial Services [Member]
HELD-TO-MATURITY
Cost or Amortized Cost	150	150
Gross Unrealized Appreciation	0	0
Gross Unrealized Depreciation	0	0
Fair Value	150	150
AVAILABLE-FOR-SALE
Cost or Amortized Cost	252,289	259,382
Gross Unrealized Appreciation	8,961	5,246
Gross Unrealized Depreciation	228	1,143
Fair Value	261,022	263,485
Mortgage-Backed Securities [Member]
HELD-TO-MATURITY
Cost or Amortized Cost	64	72
Gross Unrealized Appreciation	1	2
Gross Unrealized Depreciation	0	0
Fair Value	65	74
AVAILABLE-FOR-SALE
Cost or Amortized Cost	15,352	16,413
Gross Unrealized Appreciation	464	376
Gross Unrealized Depreciation	17	51
Fair Value	15,799	16,738
U.S. Treasury [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	23,783	21,587
Gross Unrealized Appreciation	303	100
Gross Unrealized Depreciation	3	38
Fair Value	24,083	21,649
U.S. Government Agency [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	176,116	232,808
Gross Unrealized Appreciation	3,728	2,622
Gross Unrealized Depreciation	28	2,400
Fair Value	179,816	233,030
Foreign Bonds [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	80,115	82,580
Gross Unrealized Appreciation	2,555	2,405
Gross Unrealized Depreciation	1,925	2,457
Fair Value	80,745	82,528
Public Utilities [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	215,122	213,233
Gross Unrealized Appreciation	7,624	3,701
Gross Unrealized Depreciation	826	1,251
Fair Value	221,920	215,683
Government National Mortgage Association [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	123,776	120,220
Gross Unrealized Appreciation	3,716	1,391
Gross Unrealized Depreciation	403	1,985
Fair Value	127,089	119,626
Federal Home Loan Mortgage Corporation [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	134,375	137,874
Gross Unrealized Appreciation	5,285	2,377
Gross Unrealized Depreciation	149	1,342
Fair Value	139,511	138,909
Federal National Mortgage Association [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	108,566	106,021
Gross Unrealized Appreciation	4,994	2,400
Gross Unrealized Depreciation	127	941
Fair Value	113,433	107,480
Asset-Backed Securities [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	4,789	5,461
Gross Unrealized Appreciation	238	221
Gross Unrealized Depreciation	212	16
Fair Value	4,815	5,666
Equity Securities [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	68,724	68,514
Gross Unrealized Appreciation	169,818	168,722
Gross Unrealized Depreciation	726	989
Fair Value	237,816	236,247
Common Stocks [Member] | Public Utilities [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	7,231	7,231
Gross Unrealized Appreciation	14,893	12,022
Gross Unrealized Depreciation	181	193
Fair Value	21,943	19,060
Common Stocks [Member] | Energy [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	6,514	6,103
Gross Unrealized Appreciation	6,409	5,374
Gross Unrealized Depreciation	158	266
Fair Value	12,765	11,211
Common Stocks [Member] | Industrials [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	13,252	13,251
Gross Unrealized Appreciation	32,722	31,872
Gross Unrealized Depreciation	232	313
Fair Value	45,742	44,810
Common Stocks [Member] | Consumer Goods and Services [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	10,301	10,301
Gross Unrealized Appreciation	13,978	13,017
Gross Unrealized Depreciation	5	3
Fair Value	24,274	23,315
Common Stocks [Member] | Health Care [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	7,763	7,763
Gross Unrealized Appreciation	19,108	20,454
Gross Unrealized Depreciation	0	0
Fair Value	26,871	28,217
Common Stocks [Member] | Technology, Media and Telecommunications [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	5,931	5,931
Gross Unrealized Appreciation	8,550	7,538
Gross Unrealized Depreciation	57	105
Fair Value	14,424	13,364
Common Stocks [Member] | Financial Services [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	17,289	17,392
Gross Unrealized Appreciation	74,132	78,411
Gross Unrealized Depreciation	93	109
Fair Value	91,328	95,694
Nonredeemable Preferred Stocks [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	443	542
Gross Unrealized Appreciation	26	34
Gross Unrealized Depreciation	0	0
Fair Value	$ 469	$ 576