Investments in Fixed Maturity Securities (The Amortized Cost and Fair Value of Fixed Maturity Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 1,570,480
Amortized Cost, Due after one year through five years	8,746,271
Amortized Cost, Due after five years through ten years	4,395,236
Amortized Cost, Due after ten years	2,143,998
Amortized Cost, Mortgage-backed securities	1,859,384
Total investment in fixed maturity securities, Amortized Cost	18,715,369
Fair Value, Due in one year or less	1,527,090
Fair Value, Due after one year through five years	8,299,945
Fair Value, Due after five years through ten years	4,119,772
Fair Value, Due after ten years	1,975,763
Fair Value, Mortgage-backed securities	1,669,056
Total investments in fixed maturity securities, Fair Value	17,591,626
Held-to-maturity, allowance for credit loss	$ 114	$ 387	$ 798