Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed maturities - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	$ 545,659	$ 2,264,523
Less than Twelve Months, Gross Unrealized Losses	11,011	69,336
Twelve Months or More, Fair Value	425,571	4,678,656
Twelve Months or More, Gross Unrealized Losses	17,691	432,255
Total, Fair Value	971,230	6,943,179
Total, Gross Unrealized Losses	28,702	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	112,315	367,796
Less than Twelve Months, Gross Unrealized Losses	3,984	4,844
Twelve Months or More, Fair Value	5,844	3,304,663
Twelve Months or More, Gross Unrealized Losses	25	371,649
Total, Fair Value	118,159	3,672,459
Total, Gross Unrealized Losses	4,009	376,493
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	2,215	25,764
Less than Twelve Months, Gross Unrealized Losses	7	322
Twelve Months or More, Fair Value	0	83,950
Twelve Months or More, Gross Unrealized Losses	0	2,805
Total, Fair Value	2,215	109,714
Total, Gross Unrealized Losses	7	3,127
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	0	98,437
Less than Twelve Months, Gross Unrealized Losses	0	2,346
Twelve Months or More, Fair Value	129	58,975
Twelve Months or More, Gross Unrealized Losses	12	1,810
Total, Fair Value	129	157,412
Total, Gross Unrealized Losses	12	4,156
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	34,279	627,589
Less than Twelve Months, Gross Unrealized Losses	321	28,474
Twelve Months or More, Fair Value	38,133	386,599
Twelve Months or More, Gross Unrealized Losses	1,424	22,108
Total, Fair Value	72,412	1,014,188
Total, Gross Unrealized Losses	1,745	50,582
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	45,488	269,545
Less than Twelve Months, Gross Unrealized Losses	880	7,755
Twelve Months or More, Fair Value	57,084	422,498
Twelve Months or More, Gross Unrealized Losses	2,823	15,122
Total, Fair Value	102,572	692,043
Total, Gross Unrealized Losses	3,703	22,877
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	9,706	97,367
Less than Twelve Months, Gross Unrealized Losses	18	2,521
Twelve Months or More, Fair Value	8,374	107,286
Twelve Months or More, Gross Unrealized Losses	34	4,310
Total, Fair Value	18,080	204,653
Total, Gross Unrealized Losses	52	6,831
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	223,052	373,891
Less than Twelve Months, Gross Unrealized Losses	5,389	19,217
Twelve Months or More, Fair Value	116,892	116,743
Twelve Months or More, Gross Unrealized Losses	11,494	8,554
Total, Fair Value	339,944	490,634
Total, Gross Unrealized Losses	16,883	27,771
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	118,339	358,668
Less than Twelve Months, Gross Unrealized Losses	401	3,501
Twelve Months or More, Fair Value	199,108	24,529
Twelve Months or More, Gross Unrealized Losses	1,879	264
Total, Fair Value	317,447	383,197
Total, Gross Unrealized Losses	2,280	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	0	159,638
Twelve Months or More, Gross Unrealized Losses	0	5,136
Total, Fair Value	0	205,070
Total, Gross Unrealized Losses	0	5,491
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	265	34
Less than Twelve Months, Gross Unrealized Losses	11	1
Twelve Months or More, Fair Value	7	13,775
Twelve Months or More, Gross Unrealized Losses	0	497
Total, Fair Value	272	13,809
Total, Gross Unrealized Losses	$ 11	$ 498