Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	$ 314,893	$ 2,380,194
Less than 12 months Gross Unrealized Losses	10,319	102,402
Twelve months or more Fair Value	353,368	366,405
Twelve months or more Gross Unrealized Losses	26,551	50,407
Total Fair Value	668,261	2,746,599
Total Gross Unrealized Losses	36,870	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	0	5,985
Less than 12 months Gross Unrealized Losses	0	228
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	0	0
Total Fair Value	0	5,985
Total Gross Unrealized Losses	0	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	0	77,756
Less than 12 months Gross Unrealized Losses	0	1,958
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	0	0
Total Fair Value	0	77,756
Total Gross Unrealized Losses	0	1,958
Foreign government bonds
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	2,788	44,854
Less than 12 months Gross Unrealized Losses	4	1,940
Twelve months or more Fair Value	3,454	1,813
Twelve months or more Gross Unrealized Losses	180	851
Total Fair Value	6,242	46,667
Total Gross Unrealized Losses	184	2,791
Public utilities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	1,907	323,086
Less than 12 months Gross Unrealized Losses	31	13,151
Twelve months or more Fair Value	33,821	26,094
Twelve months or more Gross Unrealized Losses	2,771	5,251
Total Fair Value	35,728	349,180
Total Gross Unrealized Losses	2,802	18,402
Redeemable preferred stock
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	670	0
Less than 12 months Gross Unrealized Losses	44	145
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	156	0
Total Fair Value	670	0
Total Gross Unrealized Losses	200	145
All other U.S. public corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	36,685	802,158
Less than 12 months Gross Unrealized Losses	617	49,343
Twelve months or more Fair Value	121,550	61,110
Twelve months or more Gross Unrealized Losses	7,454	11,799
Total Fair Value	158,235	863,268
Total Gross Unrealized Losses	8,071	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	67,986	323,218
Less than 12 months Gross Unrealized Losses	3,283	12,476
Twelve months or more Fair Value	26,786	17,103
Twelve months or more Gross Unrealized Losses	1,190	1,487
Total Fair Value	94,772	340,321
Total Gross Unrealized Losses	4,473	13,963
All other foreign public corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	782	121,662
Less than 12 months Gross Unrealized Losses	45	5,098
Twelve months or more Fair Value	8,574	6,079
Twelve months or more Gross Unrealized Losses	1,568	1,410
Total Fair Value	9,356	127,741
Total Gross Unrealized Losses	1,613	6,508
All other foreign private corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	123,488	284,191
Less than 12 months Gross Unrealized Losses	5,381	14,089
Twelve months or more Fair Value	107,454	154,791
Twelve months or more Gross Unrealized Losses	12,694	28,439
Total Fair Value	230,942	438,982
Total Gross Unrealized Losses	18,075	42,528
Asset-backed securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	79,598	249,084
Less than 12 months Gross Unrealized Losses	893	1,565
Twelve months or more Fair Value	46,920	93,675
Twelve months or more Gross Unrealized Losses	492	1,085
Total Fair Value	126,518	342,759
Total Gross Unrealized Losses	1,385	2,650
Commercial mortgage-backed securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	0	129,765
Less than 12 months Gross Unrealized Losses	0	2,350
Twelve months or more Fair Value	3,524	4,221
Twelve months or more Gross Unrealized Losses	37	79
Total Fair Value	3,524	133,986
Total Gross Unrealized Losses	37	2,429
Residential mortgage-backed securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	989	18,435
Less than 12 months Gross Unrealized Losses	21	59
Twelve months or more Fair Value	1,285	1,519
Twelve months or more Gross Unrealized Losses	9	6
Total Fair Value	2,274	19,954
Total Gross Unrealized Losses	30	65
Equity securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	2,993	35,869
Less than 12 months Gross Unrealized Losses	73	2,339
Twelve months or more Fair Value	2,806	9,281
Twelve months or more Gross Unrealized Losses	246	720
Total Fair Value	5,799	45,150
Total Gross Unrealized Losses	$ 319	$ 3,059