Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 1,035,545
Amortized Cost, Due after one year through five years	4,571,361
Amortized Cost, Due after five years through ten years	3,130,252
Amortized Cost, Due after ten years	3,208,740
Amortized Cost, Mortgaged-backed securities	1,588,225
Total investments in fixed maturity securities, Amortized Value	13,534,123	$ 13,674,692
Fair Value, Due in one year or less	1,037,210
Fair Value, Due after one year through five years	4,624,168
Fair Value, Due after five years through ten years	3,201,572
Fair Value, Due after ten years	3,199,433
Fair Value, Mortgage-backed securities	1,591,954
Total investments in fixed maturity securities, Fair Value	$ 13,654,337	$ 13,619,620