Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 1,859,431
Amortized Cost, Due after one year through five years	8,188,520
Amortized Cost, Due after five years through ten years	3,970,551
Amortized Cost, Due after ten years	2,080,064
Amortized Cost, Mortgaged-backed securities	1,598,727
Total investments in fixed maturity securities, Amortized Value	17,697,293
Fair Value, Due in one year or less	1,829,253
Fair Value, Due after one year through five years	7,879,921
Fair Value, Due after five years through ten years	3,716,098
Fair Value, Due after ten years	1,950,335
Fair Value, Mortgage-backed securities	1,481,424
Total investments in fixed maturity securities, Fair Value	16,857,031		$ 16,614,118
Cumulative effect adjustment resulting from changes in accounting principles	$ 127	$ 378	$ 387	$ 453	$ 730	$ 798