Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed maturities - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	$ 6,385,955	$ 3,514,704
Less than Twelve Months, Gross Unrealized Losses	356,259	74,056
Twelve Months or More, Fair Value	447,446	285,282
Twelve Months or More, Gross Unrealized Losses	57,136	11,189
Total, Fair Value	6,833,401	3,799,986
Total, Gross Unrealized Losses	413,395	85,245
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	679,495	73,611
Less than Twelve Months, Gross Unrealized Losses	11,532	1,182
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	679,495	73,611
Total, Gross Unrealized Losses	11,532	1,182
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	172,605	4,939
Less than Twelve Months, Gross Unrealized Losses	5,863	15
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	172,605	4,939
Total, Gross Unrealized Losses	5,863	15
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	19,396	12,504
Less than Twelve Months, Gross Unrealized Losses	432	626
Twelve Months or More, Fair Value	17,609	7,193
Twelve Months or More, Gross Unrealized Losses	1,765	96
Total, Fair Value	37,005	19,697
Total, Gross Unrealized Losses	2,197	722
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	1,410,269	695,982
Less than Twelve Months, Gross Unrealized Losses	88,098	11,466
Twelve Months or More, Fair Value	155,244	152,722
Twelve Months or More, Gross Unrealized Losses	22,229	6,277
Total, Fair Value	1,565,513	848,704
Total, Gross Unrealized Losses	110,327	17,743
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	1,422,116	854,602
Less than Twelve Months, Gross Unrealized Losses	98,951	13,858
Twelve Months or More, Fair Value	12,440	0
Twelve Months or More, Gross Unrealized Losses	1,521	0
Total, Fair Value	1,434,556	854,602
Total, Gross Unrealized Losses	100,472	13,858
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	638,720	521,995
Less than Twelve Months, Gross Unrealized Losses	44,732	7,777
Twelve Months or More, Fair Value	62,333	49,812
Twelve Months or More, Gross Unrealized Losses	6,521	1,349
Total, Fair Value	701,053	571,807
Total, Gross Unrealized Losses	51,253	9,126
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	758,009	689,156
Less than Twelve Months, Gross Unrealized Losses	88,922	33,706
Twelve Months or More, Fair Value	25,716	0
Twelve Months or More, Gross Unrealized Losses	3,168	0
Total, Fair Value	783,725	689,156
Total, Gross Unrealized Losses	92,090	33,706
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	915,803	498,944
Less than Twelve Months, Gross Unrealized Losses	8,025	757
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	915,803	498,944
Total, Gross Unrealized Losses	8,025	757
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	349,679	156,808
Less than Twelve Months, Gross Unrealized Losses	9,186	4,597
Twelve Months or More, Fair Value	174,036	75,483
Twelve Months or More, Gross Unrealized Losses	21,927	3,464
Total, Fair Value	523,715	232,291
Total, Gross Unrealized Losses	31,113	8,061
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	19,863	6,163
Less than Twelve Months, Gross Unrealized Losses	518	72
Twelve Months or More, Fair Value	68	72
Twelve Months or More, Gross Unrealized Losses	5	3
Total, Fair Value	19,931	6,235
Total, Gross Unrealized Losses	$ 523	$ 75