Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022	Dec. 31, 2021
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 1,568,436
Amortized Cost, Due after one year through five years	8,864,666
Amortized Cost, Due after five years through ten years	4,287,881
Amortized Cost, Due after ten years	2,203,296
Amortized Cost, Mortgaged-backed securities	2,064,073
Total investments in fixed maturity securities, Amortized Value	18,988,352
Fair Value, Due in one year or less	1,528,990
Fair Value, Due after one year through five years	8,503,127
Fair Value, Due after five years through ten years	4,084,883
Fair Value, Due after ten years	2,076,887
Fair Value, Mortgage-backed securities	1,901,181
Total investments in fixed maturity securities, Fair Value	18,095,068	$ 17,591,626
Cumulative effect adjustment resulting from changes in accounting principles	$ 107	$ 114	$ 378	$ 387