Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months Fair Value	$ 297,974	$ 1,917,630
Less than 12 months Gross Unrealized Losses	2,672	68,678
Twelve months or more Fair Value	419,770	284,359
Twelve months or more Gross Unrealized Losses	16,170	33,798
Total Fair Value	717,744	2,201,989
Total Gross Unrealized Losses	18,842	102,476
Fixed Maturities | U.S. Treasury securities and obligations of U.S. government authorities and agencies
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months Fair Value	0	603
Less than 12 months Gross Unrealized Losses	0	33
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	0	0
Total Fair Value	0	603
Total Gross Unrealized Losses	0	33
Fixed Maturities | Obligations of U.S. states and their political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months Fair Value	0	239,146
Less than 12 months Gross Unrealized Losses	0	6,553
Twelve months or more Fair Value	0	0
Twelve months or more Gross Unrealized Losses	0	0
Total Fair Value	0	239,146
Total Gross Unrealized Losses	0	6,553
Fixed Maturities | Foreign government bonds
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months Fair Value	16,522	81,074
Less than 12 months Gross Unrealized Losses	210	4,055
Twelve months or more Fair Value	11,959	1,690
Twelve months or more Gross Unrealized Losses	286	331
Total Fair Value	28,481	82,764
Total Gross Unrealized Losses	496	4,386
Fixed Maturities | Public utilities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months Fair Value	26,466	207,226
Less than 12 months Gross Unrealized Losses	380	7,847
Twelve months or more Fair Value	27,354	21,394
Twelve months or more Gross Unrealized Losses	678	2,672
Total Fair Value	53,820	228,620
Total Gross Unrealized Losses	1,058	10,519
Fixed Maturities | Redeemable preferred stock
Schedule of Available-for-sale Securities [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	156
Total Fair Value	0	0
Total Gross Unrealized Losses	0	156
Fixed Maturities | All other U.S. public corporate securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months Fair Value	36,100	568,763
Less than 12 months Gross Unrealized Losses	304	20,695
Twelve months or more Fair Value	102,490	73,575
Twelve months or more Gross Unrealized Losses	2,276	8,151
Total Fair Value	138,590	642,338
Total Gross Unrealized Losses	2,580	28,846
Fixed Maturities | All other U.S. private corporate securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months Fair Value	79,495	232,561
Less than 12 months Gross Unrealized Losses	781	6,082
Twelve months or more Fair Value	36,642	29,071
Twelve months or more Gross Unrealized Losses	863	1,620
Total Fair Value	116,137	261,632
Total Gross Unrealized Losses	1,644	7,702
Fixed Maturities | All other foreign public corporate securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months Fair Value	27,078	86,492
Less than 12 months Gross Unrealized Losses	190	3,188
Twelve months or more Fair Value	8,390	5,433
Twelve months or more Gross Unrealized Losses	542	939
Total Fair Value	35,468	91,925
Total Gross Unrealized Losses	732	4,127
Fixed Maturities | All other foreign private corporate securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months Fair Value	48,109	236,512
Less than 12 months Gross Unrealized Losses	472	13,604
Twelve months or more Fair Value	133,055	101,858
Twelve months or more Gross Unrealized Losses	9,126	19,851
Total Fair Value	181,164	338,370
Total Gross Unrealized Losses	9,598	33,455
Fixed Maturities | Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months Fair Value	6,351	37,355
Less than 12 months Gross Unrealized Losses	22	492
Twelve months or more Fair Value	317	49,346
Twelve months or more Gross Unrealized Losses	4	70
Total Fair Value	6,668	86,701
Total Gross Unrealized Losses	26	562
Fixed Maturities | Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months Fair Value	49,823	191,674
Less than 12 months Gross Unrealized Losses	285	5,827
Twelve months or more Fair Value	93,403	947
Twelve months or more Gross Unrealized Losses	2,254	2
Total Fair Value	143,226	192,621
Total Gross Unrealized Losses	2,539	5,829
Fixed Maturities | Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months Fair Value	8,030	36,224
Less than 12 months Gross Unrealized Losses	28	302
Twelve months or more Fair Value	6,160	1,045
Twelve months or more Gross Unrealized Losses	141	6
Total Fair Value	14,190	37,269
Total Gross Unrealized Losses	169	308
Equity securities
Schedule of Available-for-sale Securities [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	2,965
Twelve months or more Gross Unrealized Losses	0	152
Total Fair Value	0	2,965
Total Gross Unrealized Losses	$ 0	$ 152