Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 1,372,865
Amortized Cost, Due after one year through five years	5,200,384
Amortized Cost, Due after five years through ten years	3,313,415
Amortized Cost, Due after ten years	2,593,778
Amortized Cost, Mortgaged-backed securities	1,054,951
Total investments in fixed maturity securities, Amortized Value	13,535,393
Fair Value, Due in one year or less	1,364,023
Fair Value, Due after one year through five years	5,376,126
Fair Value, Due after five years through ten years	3,461,879
Fair Value, Due after ten years	2,615,189
Fair Value, Mortgage-backed securities	1,090,344
Total investments in fixed maturity securities, Fair Value	13,907,561		$ 14,194,955
Cumulative effect adjustment resulting from changes in accounting principles	$ 871	$ (948)	$ 0