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	$ 1,481,567	$ 592,004
Less than 12 months Gross Unrealized Losses	26,731	15,922
Twelve months or more Fair Value	163,757	160,128
Twelve months or more Gross Unrealized Losses	14,492	18,791
Total Fair Value	1,645,324	752,132
Total Gross Unrealized Losses	41,223	34,713
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	1,239,232	8,480
Less than 12 months Gross Unrealized Losses	20,766	107
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	0	0
Total Fair Value	1,239,232	8,480
Total Gross Unrealized Losses	20,766	107
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	6,887
Less than 12 months Gross Unrealized Losses	0	378
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	0	0
Total Fair Value	0	6,887
Total Gross Unrealized Losses	0	378
Foreign government bonds
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	2,690	13,616
Less than 12 months Gross Unrealized Losses	4	28
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	0	0
Total Fair Value	2,690	13,616
Total Gross Unrealized Losses	4	28
Public utilities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	8,168	49,104
Less than 12 months Gross Unrealized Losses	1,238	1,421
Twelve months or more Fair Value	17,325	14,217
Twelve months or more Gross Unrealized Losses	2,053	2,842
Total Fair Value	25,493	63,321
Total Gross Unrealized Losses	3,291	4,263
Redeemable preferred stock
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	2,950
Less than 12 months Gross Unrealized Losses	19
Twelve months or more Fair Value	0
Twelve months or more Gross Unrealized Losses	0
Total Fair Value	2,950
Total Gross Unrealized Losses	19
All other U.S. public corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	46,243	207,578
Less than 12 months Gross Unrealized Losses	113	6,297
Twelve months or more Fair Value	13,369	29,828
Twelve months or more Gross Unrealized Losses	1,110	1,420
Total Fair Value	59,612	237,406
Total Gross Unrealized Losses	1,223	7,717
All other U.S. private corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	50,013	84,318
Less than 12 months Gross Unrealized Losses	2,142	4,020
Twelve months or more Fair Value	20,556	3,550
Twelve months or more Gross Unrealized Losses	5,723	387
Total Fair Value	70,569	87,868
Total Gross Unrealized Losses	7,865	4,407
All other foreign public corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	14,402	76,573
Less than 12 months Gross Unrealized Losses	8	608
Twelve months or more Fair Value	9,909	0
Twelve months or more Gross Unrealized Losses	91	0
Total Fair Value	24,311	76,573
Total Gross Unrealized Losses	99	608
All other foreign private corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	58,036	38,047
Less than 12 months Gross Unrealized Losses	1,611	1,972
Twelve months or more Fair Value	64,374	85,341
Twelve months or more Gross Unrealized Losses	5,378	13,870
Total Fair Value	122,410	123,388
Total Gross Unrealized Losses	6,989	15,842
Asset-backed securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	28,462	50,195
Less than 12 months Gross Unrealized Losses	694	430
Twelve months or more Fair Value	38,224	26,359
Twelve months or more Gross Unrealized Losses	137	262
Total Fair Value	66,686	76,554
Total Gross Unrealized Losses	831	692
Commercial mortgage-backed securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	31,284	55,065
Less than 12 months Gross Unrealized Losses	135	642
Twelve months or more Fair Value	0	833
Twelve months or more Gross Unrealized Losses	0	10
Total Fair Value	31,284	55,898
Total Gross Unrealized Losses	135	652
Residential mortgage-backed securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	87	2,141
Less than 12 months Gross Unrealized Losses	1	19
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	0	0
Total Fair Value	87	2,141
Total Gross Unrealized Losses	1	19
Equity securities, available-for-sale
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	0
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	0	0
Total Fair Value	0	0
Total Gross Unrealized Losses	$ 0	$ 0