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, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 948,532
Amortized Cost, Due after one year through five years	4,526,907
Amortized Cost, Due after five years through ten years	2,958,685
Amortized Cost, Due after ten years	3,611,112
Amortized Cost, Mortgaged-backed securities	1,594,889
Total investments in fixed maturity securities, Amortized Value	13,640,125	$ 13,321,912
Fair Value, Due in one year or less	948,471
Fair Value, Due after one year through five years	4,523,008
Fair Value, Due after five years through ten years	2,959,406
Fair Value, Due after ten years	3,593,100
Fair Value, Mortgage-backed securities	1,560,669
Total investments in fixed maturity securities, Fair Value	$ 13,584,654	$ 13,566,976