Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	$ 592,004	$ 322,351
Less than 12 months Gross Unrealized Losses	15,922	5,969
Twelve months or more Fair Value	160,128	58,991
Twelve months or more Gross Unrealized Losses	18,791	1,152
Total Fair Value	752,132	381,342
Total Gross Unrealized Losses	34,713	7,121
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	8,480	2,676
Less than 12 months Gross Unrealized Losses	107	10
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	0	0
Total Fair Value	8,480	2,676
Total Gross Unrealized Losses	107	10
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	6,887	0
Less than 12 months Gross Unrealized Losses	378	0
Twelve months or more Fair Value	0	7,305
Twelve months or more Gross Unrealized Losses	0	20
Total Fair Value	6,887	7,305
Total Gross Unrealized Losses	378	20
Foreign government bonds
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	13,616	4,632
Less than 12 months Gross Unrealized Losses	28	4
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	0	0
Total Fair Value	13,616	4,632
Total Gross Unrealized Losses	28	4
Public utilities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	49,104	18,222
Less than 12 months Gross Unrealized Losses	1,421	1,321
Twelve months or more Fair Value	14,217	2,174
Twelve months or more Gross Unrealized Losses	2,842	272
Total Fair Value	63,321	20,396
Total Gross Unrealized Losses	4,263	1,593
All other U.S. public corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	207,578	144,106
Less than 12 months Gross Unrealized Losses	6,297	1,525
Twelve months or more Fair Value	29,828	6,569
Twelve months or more Gross Unrealized Losses	1,420	419
Total Fair Value	237,406	150,675
Total Gross Unrealized Losses	7,717	1,944
All other U.S. private corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	84,318	44,014
Less than 12 months Gross Unrealized Losses	4,020	518
Twelve months or more Fair Value	3,550	2,834
Twelve months or more Gross Unrealized Losses	387	10
Total Fair Value	87,868	46,848
Total Gross Unrealized Losses	4,407	528
All other foreign public corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	76,573	26,193
Less than 12 months Gross Unrealized Losses	608	35
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	0	0
Total Fair Value	76,573	26,193
Total Gross Unrealized Losses	608	35
All other foreign private corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	38,047	46,101
Less than 12 months Gross Unrealized Losses	1,972	2,384
Twelve months or more Fair Value	85,341	0
Twelve months or more Gross Unrealized Losses	13,870	0
Total Fair Value	123,388	46,101
Total Gross Unrealized Losses	15,842	2,384
Asset-backed securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	50,195	31,756
Less than 12 months Gross Unrealized Losses	430	58
Twelve months or more Fair Value	26,359	32,732
Twelve months or more Gross Unrealized Losses	262	333
Total Fair Value	76,554	64,488
Total Gross Unrealized Losses	692	391
Commercial mortgage-backed securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	55,065	4,309
Less than 12 months Gross Unrealized Losses	642	108
Twelve months or more Fair Value	833	7,377
Twelve months or more Gross Unrealized Losses	10	98
Total Fair Value	55,898	11,686
Total Gross Unrealized Losses	652	206
Residential mortgage-backed securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	2,141	342
Less than 12 months Gross Unrealized Losses	19	6
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	0	0
Total Fair Value	2,141	342
Total Gross Unrealized Losses	19	6
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