Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 867,594
Amortized Cost, Due after one year through five years	4,927,863
Amortized Cost, Due after five years through ten years	3,366,555
Amortized Cost, Due after ten years	3,194,713
Amortized Cost, Mortgaged-backed securities	1,461,616
Total investments in fixed maturity securities, Amortized Value	13,818,341
Fair Value, Due in one year or less	845,806
Fair Value, Due after one year through five years	4,921,159
Fair Value, Due after five years through ten years	3,369,995
Fair Value, Due after ten years	3,077,295
Fair Value, Mortgage-backed securities	1,464,136
Total investments in fixed maturity securities, Fair Value	13,678,391	$ 14,194,955
Cumulative effect adjustment resulting from changes in accounting principles	$ 107	$ 0