Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 852,249
Amortized Cost, Due after one year through five years	5,021,482
Amortized Cost, Due after five years through ten years	3,148,884
Amortized Cost, Due after ten years	2,863,378
Amortized Cost, Mortgaged-backed securities	1,132,437
Total investments in fixed maturity securities, Amortized Value	13,018,430
Fair Value, Due in one year or less	832,105
Fair Value, Due after one year through five years	5,151,442
Fair Value, Due after five years through ten years	3,297,837
Fair Value, Due after ten years	2,844,608
Fair Value, Mortgage-backed securities	1,170,406
Total investments in fixed maturity securities, Fair Value	13,296,398		$ 14,194,955
Cumulative effect adjustment resulting from changes in accounting principles	$ 948	$ (107)	$ 0