Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 1,778,569
Amortized Cost, Due after one year through five years	6,419,223
Amortized Cost, Due after five years through ten years	3,500,751
Amortized Cost, Due after ten years	2,359,151
Amortized Cost, Mortgaged-backed securities	1,068,556
Total investments in fixed maturity securities, Amortized Value	15,126,250
Fair Value, Due in one year or less	1,765,769
Fair Value, Due after one year through five years	6,576,943
Fair Value, Due after five years through ten years	3,592,140
Fair Value, Due after ten years	2,394,291
Fair Value, Mortgage-backed securities	1,083,049
Total investments in fixed maturity securities, Fair Value	15,412,192	$ 14,173,629
Cumulative effect adjustment resulting from changes in accounting principles	$ 730	$ 798	$ 107	$ 0