Investments In Fixed Maturity Securities Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 813,191
Amortized Cost, Due after one year through five years	4,580,074
Amortized Cost, Due after five years through ten years	3,110,432
Amortized Cost, Due after ten years	3,350,543
Amortized Cost, Mortgaged-backed securities	1,419,269
Total investments in fixed maturity securities, Amortized Value	13,273,509	$ 13,321,912
Fair Value, Due in one year or less	816,313
Fair Value, Due after one year through five years	4,621,880
Fair Value, Due after five years through ten years	3,158,810
Fair Value, Due after ten years	3,358,545
Fair Value, Mortgage-backed securities	1,400,757
Total investments in fixed maturity securities, Fair Value	$ 13,356,305	$ 13,566,976