Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 1,818,820
Amortized Cost, Due after one year through five years	9,180,726
Amortized Cost, Due after five years through ten years	4,308,202
Amortized Cost, Due after ten years	2,609,192
Amortized Cost, Mortgaged-backed securities	2,315,785
Total investments in fixed maturity securities, Amortized Value	20,232,725
Fair Value, Due in one year or less	1,742,384
Fair Value, Due after one year through five years	8,704,308
Fair Value, Due after five years through ten years	4,074,956
Fair Value, Due after ten years	2,436,157
Fair Value, Mortgage-backed securities	2,079,344
Total investments in fixed maturity securities, Fair Value	19,037,149		$ 17,591,626
Cumulative effect adjustment resulting from changes in accounting principles	$ 48	$ 53	$ 114	$ 121	$ 127	$ 387