Investments in Fixed Maturity Securities (The Amortized Cost and Fair Value of Fixed Maturity Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 1,937,329
Amortized Cost, Due after one year through five years	9,608,345
Amortized Cost, Due after five years through ten years	4,368,259
Amortized Cost, Due after ten years	2,568,038
Amortized Cost, Mortgage-backed securities	2,433,231
Total investment in fixed maturity securities, Amortized Cost	20,915,202
Fair Value, Due in one year or less	1,860,397
Fair Value, Due after one year through five years	9,327,652
Fair Value, Due after five years through ten years	4,260,217
Fair Value, Due after ten years	2,463,851
Fair Value, Mortgage-backed securities	2,269,430
Total investments in fixed maturity securities, Fair Value	20,181,547
Held-to-maturity, allowance for credit loss	$ 43	$ 114	$ 387