Investments in Fixed Maturity Securities (The Amortized Cost and Fair Value of Fixed Maturity Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 1,618,794
Amortized Cost, Due after one year through five years	5,319,638
Amortized Cost, Due after five years through ten years	3,382,528
Amortized Cost, Due after ten years	2,433,353
Amortized Cost, Mortgage-backed securities	1,000,747
Total investment in fixed maturity securities, Amortized Cost	13,755,060
Fair Value, Due in one year or less	1,612,652
Fair Value, Due after one year through five years	5,511,880
Fair Value, Due after five years through ten years	3,550,286
Fair Value, Due after ten years	2,470,983
Fair Value, Mortgage-backed securities	1,027,828
Total investments in fixed maturity securities, Fair Value	14,173,629
Held-to-maturity, allowance for credit loss	$ 798	$ 0