Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed maturities - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	$ 678,889	$ 2,264,523
Less than Twelve Months, Gross Unrealized Losses	12,288	69,336
Twelve Months or More, Fair Value	4,850,197	4,678,656
Twelve Months or More, Gross Unrealized Losses	271,515	432,255
Total, Fair Value	5,529,086	6,943,179
Total, Gross Unrealized Losses	283,803	501,591
U.S. Treasury securities and obligations of U.S. government authorities and agencies
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	0	367,796
Less than Twelve Months, Gross Unrealized Losses	0	4,844
Twelve Months or More, Fair Value	3,443,363	3,304,663
Twelve Months or More, Gross Unrealized Losses	232,980	371,649
Total, Fair Value	3,443,363	3,672,459
Total, Gross Unrealized Losses	232,980	376,493
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	0	25,764
Less than Twelve Months, Gross Unrealized Losses	0	322
Twelve Months or More, Fair Value	33,474	83,950
Twelve Months or More, Gross Unrealized Losses	153	2,805
Total, Fair Value	33,474	109,714
Total, Gross Unrealized Losses	153	3,127
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	107	98,437
Less than Twelve Months, Gross Unrealized Losses	7	2,346
Twelve Months or More, Fair Value	32,005	58,975
Twelve Months or More, Gross Unrealized Losses	155	1,810
Total, Fair Value	32,112	157,412
Total, Gross Unrealized Losses	162	4,156
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	121,372	627,589
Less than Twelve Months, Gross Unrealized Losses	2,518	28,474
Twelve Months or More, Fair Value	510,344	386,599
Twelve Months or More, Gross Unrealized Losses	15,654	22,108
Total, Fair Value	631,716	1,014,188
Total, Gross Unrealized Losses	18,172	50,582
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	57,980	269,545
Less than Twelve Months, Gross Unrealized Losses	3,187	7,755
Twelve Months or More, Fair Value	367,284	422,498
Twelve Months or More, Gross Unrealized Losses	7,437	15,122
Total, Fair Value	425,264	692,043
Total, Gross Unrealized Losses	10,624	22,877
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	16,098	97,367
Less than Twelve Months, Gross Unrealized Losses	255	2,521
Twelve Months or More, Fair Value	86,214	107,286
Twelve Months or More, Gross Unrealized Losses	1,371	4,310
Total, Fair Value	102,312	204,653
Total, Gross Unrealized Losses	1,626	6,831
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	175,953	373,891
Less than Twelve Months, Gross Unrealized Losses	3,541	19,217
Twelve Months or More, Fair Value	183,723	116,743
Twelve Months or More, Gross Unrealized Losses	10,700	8,554
Total, Fair Value	359,676	490,634
Total, Gross Unrealized Losses	14,241	27,771
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	307,379	358,668
Less than Twelve Months, Gross Unrealized Losses	2,780	3,501
Twelve Months or More, Fair Value	31,211	24,529
Twelve Months or More, Gross Unrealized Losses	459	264
Total, Fair Value	338,590	383,197
Total, Gross Unrealized Losses	3,239	3,765
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	0	45,432
Less than Twelve Months, Gross Unrealized Losses	0	355
Twelve Months or More, Fair Value	152,283	159,638
Twelve Months or More, Gross Unrealized Losses	2,260	5,136
Total, Fair Value	152,283	205,070
Total, Gross Unrealized Losses	2,260	5,491
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	0	34
Less than Twelve Months, Gross Unrealized Losses	0	1
Twelve Months or More, Fair Value	10,296	13,775
Twelve Months or More, Gross Unrealized Losses	346	497
Total, Fair Value	10,296	13,809
Total, Gross Unrealized Losses	$ 346	$ 498