Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Available For Sale Securities Debt Maturities Amortized Cost
Due in one year or less	$ 986,289
Due after one year through five years	7,797,732
Due after five year through ten years	9,311,737
Due after ten years	14,688,760
Asset and mortgage-backed securities	4,966,614
Total	37,751,132	$ 35,281,412
Available For Sale Securities Debt Maturities Fair Value
Due in one year or less	993,352
Due after one year through five years	7,888,116
Due after five year through ten years	9,407,186
Due after ten years	15,799,245
Asset and mortgage-backed securities	4,917,313
Total	$ 39,005,212	$ 38,150,820

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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