Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 1,472,865
Amortized Cost, Due after one year through five years	7,903,303
Amortized Cost, Due after five years through ten years	4,029,040
Amortized Cost, Due after ten years	1,982,273
Amortized Cost, Mortgaged-backed securities	1,450,844
Total investments in fixed maturity securities, Amortized Value	16,838,325
Fair Value, Due in one year or less	1,461,188
Fair Value, Due after one year through five years	7,756,952
Fair Value, Due after five years through ten years	3,898,727
Fair Value, Due after ten years	1,927,232
Fair Value, Mortgage-backed securities	1,389,515
Total investments in fixed maturity securities, Fair Value	16,433,614	$ 16,614,118
Cumulative effect adjustment resulting from changes in accounting principles	$ 378	$ 387	$ 730	$ 798