Investments in Fixed Maturity Securities (The Amortized Cost and Fair Value of Fixed Maturity Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 1,589,823
Amortized Cost, Due after one year through five years	7,574,884
Amortized Cost, Due after five years through ten years	4,148,118
Amortized Cost, Due after ten years	2,086,810
Amortized Cost, Mortgage-backed securities	1,071,282
Total investment in fixed maturity securities, Amortized Cost	16,470,917
Fair Value, Due in one year or less	1,586,470
Fair Value, Due after one year through five years	7,659,231
Fair Value, Due after five years through ten years	4,184,007
Fair Value, Due after ten years	2,110,874
Fair Value, Mortgage-backed securities	1,073,536
Total investments in fixed maturity securities, Fair Value	16,614,118
Held-to-maturity, allowance for credit loss	$ 387	$ 798	$ 0