Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	$ 1,917,630	$ 2,380,194
Less than 12 months Gross Unrealized Losses	68,678	102,402
Twelve months or more Fair Value	284,359	366,405
Twelve months or more Gross Unrealized Losses	33,798	50,407
Total Fair Value	2,201,989	2,746,599
Total Gross Unrealized Losses	102,476	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	603	5,985
Less than 12 months Gross Unrealized Losses	33	228
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	0	0
Total Fair Value	603	5,985
Total Gross Unrealized Losses	33	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	239,146	77,756
Less than 12 months Gross Unrealized Losses	6,553	1,958
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	0	0
Total Fair Value	239,146	77,756
Total Gross Unrealized Losses	6,553	1,958
Foreign government bonds
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	81,074	44,854
Less than 12 months Gross Unrealized Losses	4,055	1,940
Twelve months or more Fair Value	1,690	1,813
Twelve months or more Gross Unrealized Losses	331	851
Total Fair Value	82,764	46,667
Total Gross Unrealized Losses	4,386	2,791
Public utilities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	207,226	323,086
Less than 12 months Gross Unrealized Losses	7,847	13,151
Twelve months or more Fair Value	21,394	26,094
Twelve months or more Gross Unrealized Losses	2,672	5,251
Total Fair Value	228,620	349,180
Total Gross Unrealized Losses	10,519	18,402
Redeemable preferred stock
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	0	0
Less than 12 months Gross Unrealized Losses	0	145
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	156	0
Total Fair Value	0	0
Total Gross Unrealized Losses	156	145
All other U.S. public corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	568,763	802,158
Less than 12 months Gross Unrealized Losses	20,695	49,343
Twelve months or more Fair Value	73,575	61,110
Twelve months or more Gross Unrealized Losses	8,151	11,799
Total Fair Value	642,338	863,268
Total Gross Unrealized Losses	28,846	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	232,561	323,218
Less than 12 months Gross Unrealized Losses	6,082	12,476
Twelve months or more Fair Value	29,071	17,103
Twelve months or more Gross Unrealized Losses	1,620	1,487
Total Fair Value	261,632	340,321
Total Gross Unrealized Losses	7,702	13,963
All other foreign public corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	86,492	121,662
Less than 12 months Gross Unrealized Losses	3,188	5,098
Twelve months or more Fair Value	5,433	6,079
Twelve months or more Gross Unrealized Losses	939	1,410
Total Fair Value	91,925	127,741
Total Gross Unrealized Losses	4,127	6,508
All other foreign private corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	236,512	284,191
Less than 12 months Gross Unrealized Losses	13,604	14,089
Twelve months or more Fair Value	101,858	154,791
Twelve months or more Gross Unrealized Losses	19,851	28,439
Total Fair Value	338,370	438,982
Total Gross Unrealized Losses	33,455	42,528
Asset-backed securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	37,355	249,084
Less than 12 months Gross Unrealized Losses	492	1,565
Twelve months or more Fair Value	49,346	93,675
Twelve months or more Gross Unrealized Losses	70	1,085
Total Fair Value	86,701	342,759
Total Gross Unrealized Losses	562	2,650
Commercial mortgage-backed securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	191,674	129,765
Less than 12 months Gross Unrealized Losses	5,827	2,350
Twelve months or more Fair Value	947	4,221
Twelve months or more Gross Unrealized Losses	2	79
Total Fair Value	192,621	133,986
Total Gross Unrealized Losses	5,829	2,429
Residential mortgage-backed securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	36,224	18,435
Less than 12 months Gross Unrealized Losses	302	59
Twelve months or more Fair Value	1,045	1,519
Twelve months or more Gross Unrealized Losses	6	6
Total Fair Value	37,269	19,954
Total Gross Unrealized Losses	308	65
Equity securities, available-for-sale:
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	2,965	9,281
Twelve months or more Gross Unrealized Losses	152	720
Total Fair Value	2,965	45,150
Total Gross Unrealized Losses	$ 152	$ 3,059