Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 1,744,722
Amortized Cost, Due after one year through five years	8,461,536
Amortized Cost, Due after five years through ten years	4,388,799
Amortized Cost, Due after ten years	2,097,205
Amortized Cost, Mortgaged-backed securities	1,775,278
Total investments in fixed maturity securities, Amortized Value	18,467,540
Fair Value, Due in one year or less	1,691,497
Fair Value, Due after one year through five years	7,982,948
Fair Value, Due after five years through ten years	4,041,900
Fair Value, Due after ten years	1,923,272
Fair Value, Mortgage-backed securities	1,594,869
Total investments in fixed maturity securities, Fair Value	17,234,486		$ 16,614,118
Cumulative effect adjustment resulting from changes in accounting principles	$ 121	$ 127	$ 387	$ 396	$ 453	$ 798