Investments (Maturities of Fixed Maturity Securities) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Available-for-sale Securities, Debt Maturities [Abstract]
Amortized Cost, Due in one year or less	$ 13,437
Amortized Cost, Due after one year through five years	59,285
Amortized Cost, Due after five years through ten years	61,369
Amortized Cost, Due after ten years	109,711
Amortized Cost, Structured Securities	48,455
Amortized Cost, Subtotal	292,257	$ 286,069
Estimated Fair Value, Due in one year or less	13,456
Estimated Fair Value, Due after one year through five years	60,971
Estimated Fair Value, Due after five years through ten years	63,077
Estimated Fair Value, Due after ten years	119,894
Estimated Fair Value, Structured Securities	48,933
Debt Securities, Available-for-sale	$ 306,331	$ 308,931