Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 1,017,744
Amortized Cost, Due after one year through five years	4,680,578
Amortized Cost, Due after five years through ten years	3,226,328
Amortized Cost, Due after ten years	2,999,301
Amortized Cost, Mortgaged-backed securities	1,555,592
Total investments in fixed maturity securities, Amortized Value	13,479,543	$ 13,674,692
Fair Value, Due in one year or less	1,022,427
Fair Value, Due after one year through five years	4,764,795
Fair Value, Due after five years through ten years	3,347,580
Fair Value, Due after ten years	3,025,841
Fair Value, Mortgage-backed securities	1,586,329
Total investments in fixed maturity securities, Fair Value	$ 13,746,972	$ 13,619,620