Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed maturities - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	$ 9,161,351	$ 10,575,622
Less than Twelve Months, Gross Unrealized Losses	278,120	925,410
Twelve Months or More, Fair Value	10,180,149	5,707,596
Twelve Months or More, Gross Unrealized Losses	1,937,529	1,428,069
Total, Fair Value	19,341,500	16,283,218
Total, Gross Unrealized Losses	2,215,649	2,353,479
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	100,715	212,991
Less than Twelve Months, Gross Unrealized Losses	1,233	46,928
Twelve Months or More, Fair Value	219,681	62,630
Twelve Months or More, Gross Unrealized Losses	67,349	25,928
Total, Fair Value	320,396	275,621
Total, Gross Unrealized Losses	68,582	72,856
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	331,330	307,734
Less than Twelve Months, Gross Unrealized Losses	5,382	16,851
Twelve Months or More, Fair Value	146,894	61,915
Twelve Months or More, Gross Unrealized Losses	19,186	14,108
Total, Fair Value	478,224	369,649
Total, Gross Unrealized Losses	24,568	30,959
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	50,572	139,577
Less than Twelve Months, Gross Unrealized Losses	2,126	19,435
Twelve Months or More, Fair Value	214,241	111,371
Twelve Months or More, Gross Unrealized Losses	52,774	39,205
Total, Fair Value	264,813	250,948
Total, Gross Unrealized Losses	54,900	58,640
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	3,983,266	3,873,275
Less than Twelve Months, Gross Unrealized Losses	112,737	389,937
Twelve Months or More, Fair Value	3,630,814	1,979,725
Twelve Months or More, Gross Unrealized Losses	827,449	602,208
Total, Fair Value	7,614,080	5,853,000
Total, Gross Unrealized Losses	940,186	992,145
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	2,169,540	2,506,932
Less than Twelve Months, Gross Unrealized Losses	86,096	157,853
Twelve Months or More, Fair Value	1,817,855	948,686
Twelve Months or More, Gross Unrealized Losses	254,812	177,352
Total, Fair Value	3,987,395	3,455,618
Total, Gross Unrealized Losses	340,908	335,205
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	751,135	548,083
Less than Twelve Months, Gross Unrealized Losses	12,808	40,508
Twelve Months or More, Fair Value	816,331	596,437
Twelve Months or More, Gross Unrealized Losses	142,851	122,856
Total, Fair Value	1,567,466	1,144,520
Total, Gross Unrealized Losses	155,659	163,364
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	926,723	1,772,413
Less than Twelve Months, Gross Unrealized Losses	34,714	199,124
Twelve Months or More, Fair Value	2,234,946	1,479,608
Twelve Months or More, Gross Unrealized Losses	485,510	390,029
Total, Fair Value	3,161,669	3,252,021
Total, Gross Unrealized Losses	520,224	589,153
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	379,540	625,710
Less than Twelve Months, Gross Unrealized Losses	4,023	15,146
Twelve Months or More, Fair Value	611,043	289,581
Twelve Months or More, Gross Unrealized Losses	17,464	17,431
Total, Fair Value	990,583	915,291
Total, Gross Unrealized Losses	21,487	32,577
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	314,068	459,186
Less than Twelve Months, Gross Unrealized Losses	13,348	30,408
Twelve Months or More, Fair Value	456,762	176,349
Twelve Months or More, Gross Unrealized Losses	65,471	38,693
Total, Fair Value	770,830	635,535
Total, Gross Unrealized Losses	78,819	69,101
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	154,462	129,721
Less than Twelve Months, Gross Unrealized Losses	5,653	9,220
Twelve Months or More, Fair Value	31,582	1,294
Twelve Months or More, Gross Unrealized Losses	4,663	259
Total, Fair Value	186,044	131,015
Total, Gross Unrealized Losses	$ 10,316	$ 9,479