Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	$ 383,901	$ 2,380,194
Less than 12 months Gross Unrealized Losses	10,735	102,402
Twelve months or more Fair Value	272,031	366,405
Twelve months or more Gross Unrealized Losses	19,779	50,407
Total Fair Value	655,932	2,746,599
Total Gross Unrealized Losses	30,514	152,809
U.S. Treasury securities and obligations of U.S. government authorities and agencies
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	47,375	5,985
Less than 12 months Gross Unrealized Losses	407	228
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	0	0
Total Fair Value	47,375	5,985
Total Gross Unrealized Losses	407	228
Obligations of U.S. states and their political subdivisions
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	11,621	77,756
Less than 12 months Gross Unrealized Losses	132	1,958
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	0	0
Total Fair Value	11,621	77,756
Total Gross Unrealized Losses	132	1,958
Foreign government bonds
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	13,702	44,854
Less than 12 months Gross Unrealized Losses	73	1,940
Twelve months or more Fair Value	1,891	1,813
Twelve months or more Gross Unrealized Losses	131	851
Total Fair Value	15,593	46,667
Total Gross Unrealized Losses	204	2,791
Public utilities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	59,792	323,086
Less than 12 months Gross Unrealized Losses	456	13,151
Twelve months or more Fair Value	22,198	26,094
Twelve months or more Gross Unrealized Losses	2,647	5,251
Total Fair Value	81,990	349,180
Total Gross Unrealized Losses	3,103	18,402
Redeemable preferred stock
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	651	0
Less than 12 months Gross Unrealized Losses	65	145
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	156	0
Total Fair Value	651	0
Total Gross Unrealized Losses	221	145
All other U.S. public corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	82,559	802,158
Less than 12 months Gross Unrealized Losses	798	49,343
Twelve months or more Fair Value	79,597	61,110
Twelve months or more Gross Unrealized Losses	5,169	11,799
Total Fair Value	162,156	863,268
Total Gross Unrealized Losses	5,967	61,142
All other U.S. private corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	30,771	323,218
Less than 12 months Gross Unrealized Losses	2,316	12,476
Twelve months or more Fair Value	17,297	17,103
Twelve months or more Gross Unrealized Losses	537	1,487
Total Fair Value	48,068	340,321
Total Gross Unrealized Losses	2,853	13,963
All other foreign public corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	20,145	121,662
Less than 12 months Gross Unrealized Losses	499	5,098
Twelve months or more Fair Value	7,903	6,079
Twelve months or more Gross Unrealized Losses	645	1,410
Total Fair Value	28,048	127,741
Total Gross Unrealized Losses	1,144	6,508
All other foreign private corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	85,683	284,191
Less than 12 months Gross Unrealized Losses	5,551	14,089
Twelve months or more Fair Value	84,746	154,791
Twelve months or more Gross Unrealized Losses	10,264	28,439
Total Fair Value	170,429	438,982
Total Gross Unrealized Losses	15,815	42,528
Asset-backed securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	21,514	249,084
Less than 12 months Gross Unrealized Losses	428	1,565
Twelve months or more Fair Value	54,378	93,675
Twelve months or more Gross Unrealized Losses	190	1,085
Total Fair Value	75,892	342,759
Total Gross Unrealized Losses	618	2,650
Commercial mortgage-backed securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	10,088	129,765
Less than 12 months Gross Unrealized Losses	10	2,350
Twelve months or more Fair Value	2,350	4,221
Twelve months or more Gross Unrealized Losses	18	79
Total Fair Value	12,438	133,986
Total Gross Unrealized Losses	28	2,429
Residential mortgage-backed securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	0	18,435
Less than 12 months Gross Unrealized Losses	0	59
Twelve months or more Fair Value	1,671	1,519
Twelve months or more Gross Unrealized Losses	22	6
Total Fair Value	1,671	19,954
Total Gross Unrealized Losses	22	65
Equity securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	0	35,869
Less than 12 months Gross Unrealized Losses	0	2,339
Twelve months or more Fair Value	2,934	9,281
Twelve months or more Gross Unrealized Losses	183	720
Total Fair Value	2,934	45,150
Total Gross Unrealized Losses	$ 183	$ 3,059