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	$ 8,980,390	$ 5,052,071
Less than Twelve Months, Gross Unrealized Losses	683,985	94,096
Twelve Months or More, Fair Value	448,518	333,294
Twelve Months or More, Gross Unrealized Losses	75,391	19,435
Total, Fair Value	9,428,908	5,385,365
Total, Gross Unrealized Losses	759,376	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	275,075	0
Less than Twelve Months, Gross Unrealized Losses	6,836	0
Twelve Months or More, Fair Value	1,983	2,119
Twelve Months or More, Gross Unrealized Losses	235	111
Total, Fair Value	277,058	2,119
Total, Gross Unrealized Losses	7,071	111
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	141,153	104,621
Less than Twelve Months, Gross Unrealized Losses	12,308	701
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	141,153	104,621
Total, Gross Unrealized Losses	12,308	701
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	178,805	59,550
Less than Twelve Months, Gross Unrealized Losses	12,942	2,826
Twelve Months or More, Fair Value	27,715	6,473
Twelve Months or More, Gross Unrealized Losses	5,051	371
Total, Fair Value	206,520	66,023
Total, Gross Unrealized Losses	17,993	3,197
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	2,921,652	1,681,201
Less than Twelve Months, Gross Unrealized Losses	278,141	23,160
Twelve Months or More, Fair Value	201,571	180,249
Twelve Months or More, Gross Unrealized Losses	38,479	6,610
Total, Fair Value	3,123,223	1,861,450
Total, Gross Unrealized Losses	316,620	29,770
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	1,451,948	972,796
Less than Twelve Months, Gross Unrealized Losses	95,558	14,036
Twelve Months or More, Fair Value	23,847	16,409
Twelve Months or More, Gross Unrealized Losses	4,473	2,844
Total, Fair Value	1,475,795	989,205
Total, Gross Unrealized Losses	100,031	16,880
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	697,013	532,445
Less than Twelve Months, Gross Unrealized Losses	56,679	8,255
Twelve Months or More, Fair Value	79,629	29,718
Twelve Months or More, Gross Unrealized Losses	10,775	2,108
Total, Fair Value	776,642	562,163
Total, Gross Unrealized Losses	67,454	10,363
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	2,394,846	1,253,739
Less than Twelve Months, Gross Unrealized Losses	194,856	42,392
Twelve Months or More, Fair Value	57,609	57,637
Twelve Months or More, Gross Unrealized Losses	10,094	5,616
Total, Fair Value	2,452,455	1,311,376
Total, Gross Unrealized Losses	204,950	48,008
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	575,329	288,971
Less than Twelve Months, Gross Unrealized Losses	7,199	1,099
Twelve Months or More, Fair Value	6,750	0
Twelve Months or More, Gross Unrealized Losses	52	0
Total, Fair Value	582,079	288,971
Total, Gross Unrealized Losses	7,251	1,099
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	305,947	157,355
Less than Twelve Months, Gross Unrealized Losses	18,064	1,622
Twelve Months or More, Fair Value	49,414	40,689
Twelve Months or More, Gross Unrealized Losses	6,232	1,775
Total, Fair Value	355,361	198,044
Total, Gross Unrealized Losses	24,296	3,397
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	38,622	1,393
Less than Twelve Months, Gross Unrealized Losses	1,402	5
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	38,622	1,393
Total, Gross Unrealized Losses	$ 1,402	$ 5

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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