Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 886,582
Amortized Cost, Due after one year through five years	5,013,048
Amortized Cost, Due after five years through ten years	3,220,188
Amortized Cost, Due after ten years	3,035,535
Amortized Cost, Mortgaged-backed securities	1,689,259
Total investments in fixed maturity securities, Amortized Value	13,844,612	$ 13,674,692
Fair Value, Due in one year or less	870,937
Fair Value, Due after one year through five years	5,072,203
Fair Value, Due after five years through ten years	3,360,359
Fair Value, Due after ten years	3,077,072
Fair Value, Mortgage-backed securities	1,720,170
Total investments in fixed maturity securities, Fair Value	$ 14,100,741	$ 13,619,620