Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Available For Sale Securities Debt Maturities Amortized Cost
Due in one year or less	$ 861,051
Due after one year through five years	7,421,897
Due after five year through ten years	9,514,341
Due after ten years	11,247,037
Asset and mortgage-backed securities	4,694,008
Total	33,738,334	$ 30,211,787
Available For Sale Securities Debt Maturities Fair Value
Due in one year or less	868,405
Due after one year through five years	7,696,337
Due after five year through ten years	10,016,984
Due after ten years	13,000,532
Asset and mortgage-backed securities	4,763,168
Total	$ 36,345,426	$ 32,093,625