Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	$ 246,182	$ 592,004
Less than 12 months Gross Unrealized Losses	14,086	15,922
Twelve months or more Fair Value	170,246	160,128
Twelve months or more Gross Unrealized Losses	12,318	18,791
Total Fair Value	416,428	752,132
Total Gross Unrealized Losses	26,404	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	0	8,480
Less than 12 months Gross Unrealized Losses	0	107
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	0	0
Total Fair Value	0	8,480
Total Gross Unrealized Losses	0	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	6,072	13,616
Less than 12 months Gross Unrealized Losses	10	28
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	0	0
Total Fair Value	6,072	13,616
Total Gross Unrealized Losses	10	28
Public utilities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	8,393	49,104
Less than 12 months Gross Unrealized Losses	442	1,421
Twelve months or more Fair Value	15,296	14,217
Twelve months or more Gross Unrealized Losses	1,650	2,842
Total Fair Value	23,689	63,321
Total Gross Unrealized Losses	2,092	4,263
All other U.S. public corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	52,075	207,578
Less than 12 months Gross Unrealized Losses	4,182	6,297
Twelve months or more Fair Value	37,953	29,828
Twelve months or more Gross Unrealized Losses	828	1,420
Total Fair Value	90,028	237,406
Total Gross Unrealized Losses	5,010	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	65,504	84,318
Less than 12 months Gross Unrealized Losses	4,935	4,020
Twelve months or more Fair Value	1,772	3,550
Twelve months or more Gross Unrealized Losses	295	387
Total Fair Value	67,276	87,868
Total Gross Unrealized Losses	5,230	4,407
All other foreign public corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	13,602	76,573
Less than 12 months Gross Unrealized Losses	34	608
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	0	0
Total Fair Value	13,602	76,573
Total Gross Unrealized Losses	34	608
All other foreign private corporate securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	33,779	38,047
Less than 12 months Gross Unrealized Losses	3,137	1,972
Twelve months or more Fair Value	88,232	85,341
Twelve months or more Gross Unrealized Losses	9,240	13,870
Total Fair Value	122,011	123,388
Total Gross Unrealized Losses	12,377	15,842
Asset-backed securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	66,304	50,195
Less than 12 months Gross Unrealized Losses	1,336	430
Twelve months or more Fair Value	26,332	26,359
Twelve months or more Gross Unrealized Losses	302	262
Total Fair Value	92,636	76,554
Total Gross Unrealized Losses	1,638	692
Commercial mortgage-backed securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	197	55,065
Less than 12 months Gross Unrealized Losses	1	642
Twelve months or more Fair Value	661	833
Twelve months or more Gross Unrealized Losses	3	10
Total Fair Value	858	55,898
Total Gross Unrealized Losses	4	652
Residential mortgage-backed securities
Duration Of Unrealized Losses On Fixed Maturities Investments [Line Items]
Less than twelve months Fair Value	256	2,141
Less than 12 months Gross Unrealized Losses	9	19
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	0	0
Total Fair Value	256	2,141
Total Gross Unrealized Losses	9	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