Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Available For Sale Securities Debt Maturities Amortized Cost
Due in one year or less	$ 910,821
Due after one year through five years	7,339,134
Due after five year through ten years	9,581,912
Due after ten years	11,438,508
Asset and mortgage-backed securities	4,619,593
Total	33,889,968	$ 30,211,787
Available For Sale Securities Debt Maturities Fair Value
Due in one year or less	915,586
Due after one year through five years	7,618,692
Due after five year through ten years	10,090,704
Due after ten years	13,069,565
Asset and mortgage-backed securities	4,687,195
Total	$ 36,381,742	$ 32,093,625