Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Amortized Cost
Due in one year or less	$ 1,729,699
Due after one year through five years	9,365,511
Due after five years through ten years	6,980,612
Due after ten years	7,433,790
Amortized Cost	29,821,440	$ 27,538,066
Fair Value
Due in one year or less	1,690,127
Due after one year through five years	9,041,701
Due after five years through ten years	6,650,110
Due after ten years	6,387,440
Fair Value	28,031,484	26,131,780
Asset-backed securities
Amortized Cost
Debt securities, available-for-sale, maturity, without single maturity date, amortized cost	3,024,574
Amortized Cost	3,024,574	2,016,028
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	3,042,804
Fair Value	3,042,804	2,027,550
Commercial mortgage-backed securities
Amortized Cost
Debt securities, available-for-sale, maturity, without single maturity date, amortized cost	893,912
Amortized Cost	893,912	913,347
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	834,517
Fair Value	834,517	851,778
Residential mortgage-backed securities
Amortized Cost
Debt securities, available-for-sale, maturity, without single maturity date, amortized cost	393,342
Amortized Cost	393,342	399,542
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	384,785
Fair Value	$ 384,785	$ 396,070

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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