Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed maturities - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Less than twelve months, Fair Value	$ 2,471,059	$ 297,974
Less than twelve Months, Gross Unrealized Losses	84,054	2,672
Twelve months or more, Fair Value	390,763	419,770
Twelve months or more, Gross Unrealized Losses	35,220	16,170
Total, Fair Value	2,861,822	717,744
Total, Gross Unrealized Losses	119,274	18,842
US Treasury securities and obligations of U.S. government authorities and agencies
Debt Securities, Available-for-sale [Line Items]
Less than twelve months, Fair Value	617	0
Less than twelve Months, Gross Unrealized Losses	19	0
Twelve months or more, Fair Value	0	0
Twelve months or more, Gross Unrealized Losses	0	0
Total, Fair Value	617	0
Total, Gross Unrealized Losses	19	0
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than twelve months, Fair Value	219,185	0
Less than twelve Months, Gross Unrealized Losses	3,893	0
Twelve months or more, Fair Value	0	0
Twelve months or more, Gross Unrealized Losses	0	0
Total, Fair Value	219,185	0
Total, Gross Unrealized Losses	3,893	0
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Less than twelve months, Fair Value	118,010	16,522
Less than twelve Months, Gross Unrealized Losses	4,515	210
Twelve months or more, Fair Value	20,274	11,959
Twelve months or more, Gross Unrealized Losses	1,663	286
Total, Fair Value	138,284	28,481
Total, Gross Unrealized Losses	6,178	496
Public utilities
Debt Securities, Available-for-sale [Line Items]
Less than twelve months, Fair Value	334,468	26,466
Less than twelve Months, Gross Unrealized Losses	12,808	380
Twelve months or more, Fair Value	29,857	27,354
Twelve months or more, Gross Unrealized Losses	2,766	678
Total, Fair Value	364,325	53,820
Total, Gross Unrealized Losses	15,574	1,058
Redeemable preferred stock
Debt Securities, Available-for-sale [Line Items]
Less than twelve months, Fair Value	1,789	0
Less than twelve Months, Gross Unrealized Losses	20	0
Twelve months or more, Fair Value	0	0
Twelve months or more, Gross Unrealized Losses	0	0
Total, Fair Value	1,789	0
Total, Gross Unrealized Losses	20	0
All other U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than twelve months, Fair Value	618,747	36,100
Less than twelve Months, Gross Unrealized Losses	26,700	304
Twelve months or more, Fair Value	106,631	102,490
Twelve months or more, Gross Unrealized Losses	8,522	2,276
Total, Fair Value	725,378	138,590
Total, Gross Unrealized Losses	35,222	2,580
All other U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than twelve months, Fair Value	383,595	79,495
Less than twelve Months, Gross Unrealized Losses	10,490	781
Twelve months or more, Fair Value	81,864	36,642
Twelve months or more, Gross Unrealized Losses	4,358	863
Total, Fair Value	465,459	116,137
Total, Gross Unrealized Losses	14,848	1,644
All other foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than twelve months, Fair Value	169,812	27,078
Less than twelve Months, Gross Unrealized Losses	6,447	190
Twelve months or more, Fair Value	15,813	8,390
Twelve months or more, Gross Unrealized Losses	1,647	542
Total, Fair Value	185,625	35,468
Total, Gross Unrealized Losses	8,094	732
All other foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than twelve months, Fair Value	378,179	48,109
Less than twelve Months, Gross Unrealized Losses	14,147	472
Twelve months or more, Fair Value	75,531	133,055
Twelve months or more, Gross Unrealized Losses	12,374	9,126
Total, Fair Value	453,710	181,164
Total, Gross Unrealized Losses	26,521	9,598
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than twelve months, Fair Value	38,396	6,351
Less than twelve Months, Gross Unrealized Losses	136	22
Twelve months or more, Fair Value	4,052	317
Twelve months or more, Gross Unrealized Losses	87	4
Total, Fair Value	42,448	6,668
Total, Gross Unrealized Losses	223	26
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than twelve months, Fair Value	152,005	49,823
Less than twelve Months, Gross Unrealized Losses	3,570	285
Twelve months or more, Fair Value	44,529	93,403
Twelve months or more, Gross Unrealized Losses	3,130	2,254
Total, Fair Value	196,534	143,226
Total, Gross Unrealized Losses	6,700	2,539
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than twelve months, Fair Value	56,256	8,030
Less than twelve Months, Gross Unrealized Losses	1,309	28
Twelve months or more, Fair Value	12,212	6,160
Twelve months or more, Gross Unrealized Losses	673	141
Total, Fair Value	68,468	14,190
Total, Gross Unrealized Losses	$ 1,982	$ 169