Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed maturities - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	$ 259,895	$ 2,264,523
Less than Twelve Months, Gross Unrealized Losses	4,242	69,336
Twelve Months or More, Fair Value	1,952,743	4,678,656
Twelve Months or More, Gross Unrealized Losses	86,046	432,255
Total, Fair Value	2,212,638	6,943,179
Total, Gross Unrealized Losses	90,288	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	1,340,698	3,304,663
Twelve Months or More, Gross Unrealized Losses	69,324	371,649
Total, Fair Value	1,340,698	3,672,459
Total, Gross Unrealized Losses	69,324	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	0	83,950
Twelve Months or More, Gross Unrealized Losses	0	2,805
Total, Fair Value	0	109,714
Total, Gross Unrealized Losses	0	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	131	58,975
Twelve Months or More, Gross Unrealized Losses	16	1,810
Total, Fair Value	131	157,412
Total, Gross Unrealized Losses	16	4,156
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	13,122	627,589
Less than Twelve Months, Gross Unrealized Losses	711	28,474
Twelve Months or More, Fair Value	92,531	386,599
Twelve Months or More, Gross Unrealized Losses	2,828	22,108
Total, Fair Value	105,653	1,014,188
Total, Gross Unrealized Losses	3,539	50,582
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	32,982	269,545
Less than Twelve Months, Gross Unrealized Losses	1,352	7,755
Twelve Months or More, Fair Value	122,846	422,498
Twelve Months or More, Gross Unrealized Losses	2,932	15,122
Total, Fair Value	155,828	692,043
Total, Gross Unrealized Losses	4,284	22,877
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	11,590	97,367
Less than Twelve Months, Gross Unrealized Losses	325	2,521
Twelve Months or More, Fair Value	19,845	107,286
Twelve Months or More, Gross Unrealized Losses	194	4,310
Total, Fair Value	31,435	204,653
Total, Gross Unrealized Losses	519	6,831
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	78,867	373,891
Less than Twelve Months, Gross Unrealized Losses	1,336	19,217
Twelve Months or More, Fair Value	143,872	116,743
Twelve Months or More, Gross Unrealized Losses	8,764	8,554
Total, Fair Value	222,739	490,634
Total, Gross Unrealized Losses	10,100	27,771
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	123,334	358,668
Less than Twelve Months, Gross Unrealized Losses	518	3,501
Twelve Months or More, Fair Value	185,263	24,529
Twelve Months or More, Gross Unrealized Losses	1,781	264
Total, Fair Value	308,597	383,197
Total, Gross Unrealized Losses	2,299	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	40,513	159,638
Twelve Months or More, Gross Unrealized Losses	74	5,136
Total, Fair Value	40,513	205,070
Total, Gross Unrealized Losses	74	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	7,044	13,775
Twelve Months or More, Gross Unrealized Losses	133	497
Total, Fair Value	7,044	13,809
Total, Gross Unrealized Losses	$ 133	$ 498