Investments In Fixed Maturity Securities Investments in Fixed Maturity Securities (Amortized cost and fair value of fixed maturity securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 745,010
Amortized Cost, Due after one year through five years	5,089,768
Amortized Cost, Due after five years through ten years	3,248,254
Amortized Cost, Due after ten years	3,145,963
Amortized Cost, Mortgaged-backed securities	1,330,116
Total investments in fixed maturity securities, Amortized Value	13,559,111	$ 12,958,335
Fair Value, Due in one year or less	750,479
Fair Value, Due after one year through five years	5,210,268
Fair Value, Due after five years through ten years	3,396,461
Fair Value, Due after ten years	3,197,551
Fair Value, Mortgage-backed securities	1,335,595
Total investments in fixed maturity securities, Fair Value	$ 13,890,354	$ 13,204,814