Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 1,608,825
Amortized Cost, Due after one year through five years	8,939,735
Amortized Cost, Due after five years through ten years	4,080,542
Amortized Cost, Due after ten years	2,377,874
Amortized Cost, Mortgaged-backed securities	2,097,319
Total investments in fixed maturity securities, Amortized Value	19,104,295
Fair Value, Due in one year or less	1,560,272
Fair Value, Due after one year through five years	8,491,984
Fair Value, Due after five years through ten years	3,870,342
Fair Value, Due after ten years	2,229,004
Fair Value, Mortgage-backed securities	1,906,903
Total investments in fixed maturity securities, Fair Value	18,058,505		$ 17,591,626
Cumulative effect adjustment resulting from changes in accounting principles	$ 53	$ 107	$ 114	$ 127	$ 378	$ 387