Summary of Investments (Fair Value of Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
HELD-TO-MATURITY
Cost or Amortized Cost	$ 658	$ 672
Fair Value	660	675
AVAILABLE-FOR-SALE
Cost or Amortized Cost	2,880,838	2,861,583
Gross Unrealized Appreciation	316,099	227,716
Gross Unrealized Depreciation	5,393	28,091
Fair Value	3,191,544	3,061,208
Fixed Maturities [Member]
HELD-TO-MATURITY
Cost or Amortized Cost	658	672
Gross Unrealized Appreciation	2	3
Gross Unrealized Depreciation	0	0
Fair Value	660	675
AVAILABLE-FOR-SALE
Cost or Amortized Cost	2,812,091	2,793,069
Gross Unrealized Appreciation	135,079	58,994
Gross Unrealized Depreciation	4,806	27,102
Fair Value	2,942,364	2,824,961
Corporate Bonds [Member] | Energy [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	110,618	116,800
Gross Unrealized Appreciation	3,361	1,032
Gross Unrealized Depreciation	754	4,713
Fair Value	113,225	113,119
Corporate Bonds [Member] | Industrials [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	240,772	227,589
Gross Unrealized Appreciation	12,143	3,329
Gross Unrealized Depreciation	1,821	6,663
Fair Value	251,094	224,255
Corporate Bonds [Member] | Consumer Goods and Services [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	180,924	172,529
Gross Unrealized Appreciation	9,191	2,844
Gross Unrealized Depreciation	1	776
Fair Value	190,114	174,597
Corporate Bonds [Member] | Health Care [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	84,390	92,132
Gross Unrealized Appreciation	4,975	2,168
Gross Unrealized Depreciation	0	791
Fair Value	89,365	93,509
Corporate Bonds [Member] | Technology, Media and Telecommunications [Member]
HELD-TO-MATURITY
Cost or Amortized Cost	450	450
Gross Unrealized Appreciation	1	1
Gross Unrealized Depreciation	0	0
Fair Value	451	451
AVAILABLE-FOR-SALE
Cost or Amortized Cost	139,573	142,431
Gross Unrealized Appreciation	6,186	1,972
Gross Unrealized Depreciation	438	2,003
Fair Value	145,321	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	269,290	259,382
Gross Unrealized Appreciation	12,751	5,246
Gross Unrealized Depreciation	60	1,143
Fair Value	281,981	263,485
Mortgage-Backed Securities [Member]
HELD-TO-MATURITY
Cost or Amortized Cost	58	72
Gross Unrealized Appreciation	1	2
Gross Unrealized Depreciation	0	0
Fair Value	59	74
AVAILABLE-FOR-SALE
Cost or Amortized Cost	19,609	16,413
Gross Unrealized Appreciation	484	376
Gross Unrealized Depreciation	21	51
Fair Value	20,072	16,738
U.S. Treasury [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	22,831	21,587
Gross Unrealized Appreciation	386	100
Gross Unrealized Depreciation	0	38
Fair Value	23,217	21,649
U.S. Government Agency [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	102,445	232,808
Gross Unrealized Appreciation	3,899	2,622
Gross Unrealized Depreciation	0	2,400
Fair Value	106,344	233,030
States, Municipalities and Political Subdivisions [Member] | General Obligations [Member] | Midwest [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	145,607	160,484
Gross Unrealized Appreciation	6,500	4,990
Gross Unrealized Depreciation	0	18
Fair Value	152,107	165,456
States, Municipalities and Political Subdivisions [Member] | General Obligations [Member] | Northeast [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	61,256	56,449
Gross Unrealized Appreciation	3,386	1,996
Gross Unrealized Depreciation	0	0
Fair Value	64,642	58,445
States, Municipalities and Political Subdivisions [Member] | General Obligations [Member] | South [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	133,105	125,565
Gross Unrealized Appreciation	5,652	3,358
Gross Unrealized Depreciation	0	134
Fair Value	138,757	128,789
States, Municipalities and Political Subdivisions [Member] | General Obligations [Member] | West [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	114,463	103,721
Gross Unrealized Appreciation	5,871	3,160
Gross Unrealized Depreciation	0	67
Fair Value	120,334	106,814
States, Municipalities and Political Subdivisions [Member] | Special Revenue [Member] | Midwest [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	163,531	152,780
Gross Unrealized Appreciation	8,733	4,956
Gross Unrealized Depreciation	0	30
Fair Value	172,264	157,706
States, Municipalities and Political Subdivisions [Member] | Special Revenue [Member] | Northeast [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	46,855	23,892
Gross Unrealized Appreciation	2,442	919
Gross Unrealized Depreciation	0	212
Fair Value	49,297	24,599
States, Municipalities and Political Subdivisions [Member] | Special Revenue [Member] | South [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	193,752	144,183
Gross Unrealized Appreciation	10,043	4,281
Gross Unrealized Depreciation	14	27
Fair Value	203,781	148,437
States, Municipalities and Political Subdivisions [Member] | Special Revenue [Member] | West [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	93,041	78,935
Gross Unrealized Appreciation	5,090	3,150
Gross Unrealized Depreciation	0	44
Fair Value	98,131	82,041
Foreign Bonds [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	79,744	82,580
Gross Unrealized Appreciation	3,428	2,405
Gross Unrealized Depreciation	1,012	2,457
Fair Value	82,160	82,528
Public Utilities [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	218,538	213,233
Gross Unrealized Appreciation	11,156	3,701
Gross Unrealized Depreciation	281	1,251
Fair Value	229,413	215,683
Government National Mortgage Association [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	132,500	120,220
Gross Unrealized Appreciation	5,667	1,391
Gross Unrealized Depreciation	173	1,985
Fair Value	137,994	119,626
Federal Home Loan Mortgage Corporation [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	144,224	137,874
Gross Unrealized Appreciation	7,012	2,377
Gross Unrealized Depreciation	49	1,342
Fair Value	151,187	138,909
Federal National Mortgage Association [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	110,380	106,021
Gross Unrealized Appreciation	6,457	2,400
Gross Unrealized Depreciation	85	941
Fair Value	116,752	107,480
Asset-Backed Securities [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	4,643	5,461
Gross Unrealized Appreciation	266	221
Gross Unrealized Depreciation	97	16
Fair Value	4,812	5,666
Equity Securities [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	68,747	68,514
Gross Unrealized Appreciation	181,020	168,722
Gross Unrealized Depreciation	587	989
Fair Value	249,180	236,247
Common Stocks [Member] | Public Utilities [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	7,231	7,231
Gross Unrealized Appreciation	16,213	12,022
Gross Unrealized Depreciation	161	193
Fair Value	23,283	19,060
Common Stocks [Member] | Energy [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	6,514	6,103
Gross Unrealized Appreciation	7,914	5,374
Gross Unrealized Depreciation	46	266
Fair Value	14,382	11,211
Common Stocks [Member] | Industrials [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	13,252	13,251
Gross Unrealized Appreciation	33,200	31,872
Gross Unrealized Depreciation	207	313
Fair Value	46,245	44,810
Common Stocks [Member] | Consumer Goods and Services [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	10,324	10,301
Gross Unrealized Appreciation	15,210	13,017
Gross Unrealized Depreciation	45	3
Fair Value	25,489	23,315
Common Stocks [Member] | Health Care [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	7,763	7,763
Gross Unrealized Appreciation	20,205	20,454
Gross Unrealized Depreciation	0	0
Fair Value	27,968	28,217
Common Stocks [Member] | Technology, Media and Telecommunications [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	5,931	5,931
Gross Unrealized Appreciation	9,049	7,538
Gross Unrealized Depreciation	55	105
Fair Value	14,925	13,364
Common Stocks [Member] | Financial Services [Member]
AVAILABLE-FOR-SALE
Cost or Amortized Cost	17,289	17,392
Gross Unrealized Appreciation	79,203	78,411
Gross Unrealized Depreciation	73	109
Fair Value	96,419	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