Investments (Maturities of Fixed Maturity Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Available-for-sale Securities, Debt Maturities [Abstract]
Amortized Cost, Due in one year or less	$ 14,290
Amortized Cost, Due after one year through five years	59,495
Amortized Cost, Due after five years through ten years	61,007
Amortized Cost, Due after ten years	109,511
Amortized Cost, Structured Securities	48,965
Amortized Cost, Subtotal	293,268	$ 286,069
Estimated Fair Value, Due in one year or less	14,336
Estimated Fair Value, Due after one year through five years	60,995
Estimated Fair Value, Due after five years through ten years	62,655
Estimated Fair Value, Due after ten years	117,673
Estimated Fair Value, Structured Securities	49,181
Debt Securities, Available-for-sale	$ 304,840	$ 308,931