Investments In Fixed Maturity Securities Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 841,253
Amortized Cost, Due after one year through five years	4,392,395
Amortized Cost, Due after five years through ten years	3,042,177
Amortized Cost, Due after ten years	3,413,887
Amortized Cost, Mortgaged-backed securities	1,496,244
Total investments in fixed maturity securities, Amortized Value	13,185,956	$ 13,321,912
Fair Value, Due in one year or less	849,860
Fair Value, Due after one year through five years	4,428,769
Fair Value, Due after five years through ten years	3,074,332
Fair Value, Due after ten years	3,407,047
Fair Value, Mortgage-backed securities	1,471,384
Total investments in fixed maturity securities, Fair Value	$ 13,231,392	$ 13,566,976