Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed maturities - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, Fair Value	$ 1,419,400	$ 297,974
Less than twelve Months, Gross Unrealized Losses	28,136	2,672
Twelve months or more, Fair Value	319,664	419,770
Twelve months or more, Gross Unrealized Losses	23,341	16,170
Total, Fair Value	1,739,064	717,744
Total, Gross Unrealized Losses	51,477	18,842
US 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	624	0
Less than twelve Months, Gross Unrealized Losses	10	0
Twelve months or more, Fair Value	0	0
Twelve months or more, Gross Unrealized Losses	0	0
Total, Fair Value	624	0
Total, Gross Unrealized Losses	10	0
Obligations of U.S. states and their political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, Fair Value	60,950	0
Less than twelve Months, Gross Unrealized Losses	492	0
Twelve months or more, Fair Value	0	0
Twelve months or more, Gross Unrealized Losses	0	0
Total, Fair Value	60,950	0
Total, Gross Unrealized Losses	492	0
Foreign government bonds
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, Fair Value	108,388	16,522
Less than twelve Months, Gross Unrealized Losses	3,083	210
Twelve months or more, Fair Value	11,398	11,959
Twelve months or more, Gross Unrealized Losses	812	286
Total, Fair Value	119,786	28,481
Total, Gross Unrealized Losses	3,895	496
Public utilities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, Fair Value	135,788	26,466
Less than twelve Months, Gross Unrealized Losses	3,081	380
Twelve months or more, Fair Value	17,852	27,354
Twelve months or more, Gross Unrealized Losses	1,317	678
Total, Fair Value	153,640	53,820
Total, Gross Unrealized Losses	4,398	1,058
All other U.S. public corporate securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, Fair Value	382,978	36,100
Less than twelve Months, Gross Unrealized Losses	9,187	304
Twelve months or more, Fair Value	90,933	102,490
Twelve months or more, Gross Unrealized Losses	5,649	2,276
Total, Fair Value	473,911	138,590
Total, Gross Unrealized Losses	14,836	2,580
All other U.S. private corporate securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, Fair Value	332,751	79,495
Less than twelve Months, Gross Unrealized Losses	4,299	781
Twelve months or more, Fair Value	31,181	36,642
Twelve months or more, Gross Unrealized Losses	1,605	863
Total, Fair Value	363,932	116,137
Total, Gross Unrealized Losses	5,904	1,644
All other foreign public corporate securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, Fair Value	101,487	27,078
Less than twelve Months, Gross Unrealized Losses	2,748	190
Twelve months or more, Fair Value	6,782	8,390
Twelve months or more, Gross Unrealized Losses	725	542
Total, Fair Value	108,269	35,468
Total, Gross Unrealized Losses	3,473	732
All other foreign private corporate securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, Fair Value	151,956	48,109
Less than twelve Months, Gross Unrealized Losses	2,372	472
Twelve months or more, Fair Value	64,751	133,055
Twelve months or more, Gross Unrealized Losses	7,978	9,126
Total, Fair Value	216,707	181,164
Total, Gross Unrealized Losses	10,350	9,598
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, Fair Value	13,376	6,351
Less than twelve Months, Gross Unrealized Losses	107	22
Twelve months or more, Fair Value	308	317
Twelve months or more, Gross Unrealized Losses	4	4
Total, Fair Value	13,684	6,668
Total, Gross Unrealized Losses	111	26
Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, Fair Value	101,300	49,823
Less than twelve Months, Gross Unrealized Losses	2,114	285
Twelve months or more, Fair Value	90,648	93,403
Twelve months or more, Gross Unrealized Losses	4,983	2,254
Total, Fair Value	191,948	143,226
Total, Gross Unrealized Losses	7,097	2,539
Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less than twelve months, Fair Value	29,802	8,030
Less than twelve Months, Gross Unrealized Losses	643	28
Twelve months or more, Fair Value	5,811	6,160
Twelve months or more, Gross Unrealized Losses	268	141
Total, Fair Value	35,613	14,190
Total, Gross Unrealized Losses	$ 911	$ 169