Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Amortized Cost
Due in one year or less	$ 753,797
Due after one year through five years	8,340,245
Due after five years through ten years	6,783,822
Due after ten years	7,437,553
Amortized Cost	26,682,558	$ 21,311,087
Fair Value
Due in one year or less	735,127
Due after one year through five years	7,851,028
Due after five years through ten years	6,071,535
Due after ten years	5,897,149
Fair Value	23,798,355	$ 19,025,401
Asset-backed securities
Amortized Cost
Debt securities, available-for-sale, maturity, without single maturity date, amortized cost	2,037,295
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	2,029,708
Commercial mortgage-backed securities
Amortized Cost
Debt securities, available-for-sale, maturity, without single maturity date, amortized cost	926,653
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	832,787
Residential mortgage-backed securities
Amortized Cost
Debt securities, available-for-sale, maturity, without single maturity date, amortized cost	403,193
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	$ 381,021

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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