Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed maturities - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	$ 11,793,852	$ 5,052,071
Less than Twelve Months, Gross Unrealized Losses	1,644,884	94,096
Twelve Months or More, Fair Value	618,356	333,294
Twelve Months or More, Gross Unrealized Losses	157,625	19,435
Total, Fair Value	12,412,208	5,385,365
Total, Gross Unrealized Losses	1,802,509	113,531
US Treasury securities and obligations of U.S. government authorities and agencies
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	446,399	0
Less than Twelve Months, Gross Unrealized Losses	44,627	0
Twelve Months or More, Fair Value	1,909	2,119
Twelve Months or More, Gross Unrealized Losses	298	111
Total, Fair Value	448,308	2,119
Total, Gross Unrealized Losses	44,925	111
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	332,051	104,621
Less than Twelve Months, Gross Unrealized Losses	20,633	701
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	332,051	104,621
Total, Gross Unrealized Losses	20,633	701
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	222,423	59,550
Less than Twelve Months, Gross Unrealized Losses	42,170	2,826
Twelve Months or More, Fair Value	23,269	6,473
Twelve Months or More, Gross Unrealized Losses	9,688	371
Total, Fair Value	245,692	66,023
Total, Gross Unrealized Losses	51,858	3,197
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	4,343,243	1,681,201
Less than Twelve Months, Gross Unrealized Losses	670,173	23,160
Twelve Months or More, Fair Value	170,150	180,249
Twelve Months or More, Gross Unrealized Losses	66,139	6,610
Total, Fair Value	4,513,393	1,861,450
Total, Gross Unrealized Losses	736,312	29,770
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	2,062,339	972,796
Less than Twelve Months, Gross Unrealized Losses	215,192	14,036
Twelve Months or More, Fair Value	100,094	16,409
Twelve Months or More, Gross Unrealized Losses	13,676	2,844
Total, Fair Value	2,162,433	989,205
Total, Gross Unrealized Losses	228,868	16,880
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	825,007	532,445
Less than Twelve Months, Gross Unrealized Losses	119,741	8,255
Twelve Months or More, Fair Value	60,464	29,718
Twelve Months or More, Gross Unrealized Losses	16,277	2,108
Total, Fair Value	885,471	562,163
Total, Gross Unrealized Losses	136,018	10,363
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	2,441,224	1,253,739
Less than Twelve Months, Gross Unrealized Losses	466,582	42,392
Twelve Months or More, Fair Value	162,515	57,637
Twelve Months or More, Gross Unrealized Losses	36,921	5,616
Total, Fair Value	2,603,739	1,311,376
Total, Gross Unrealized Losses	503,503	48,008
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	592,720	288,971
Less than Twelve Months, Gross Unrealized Losses	26,070	1,099
Twelve Months or More, Fair Value	53,576	0
Twelve Months or More, Gross Unrealized Losses	5,406	0
Total, Fair Value	646,296	288,971
Total, Gross Unrealized Losses	31,476	1,099
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	490,544	157,355
Less than Twelve Months, Gross Unrealized Losses	36,875	1,622
Twelve Months or More, Fair Value	46,379	40,689
Twelve Months or More, Gross Unrealized Losses	9,220	1,775
Total, Fair Value	536,923	198,044
Total, Gross Unrealized Losses	46,095	3,397
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	37,902	1,393
Less than Twelve Months, Gross Unrealized Losses	2,821	5
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	37,902	1,393
Total, Gross Unrealized Losses	$ 2,821	$ 5