Investments (Maturities of Fixed Maturity Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale Securities, Debt Maturities [Abstract]
Amortized Cost, Due in one year or less	$ 1,801
Amortized Cost, Due after one year through five years	8,096
Amortized Cost, Due after five years through ten years	8,570
Amortized Cost, Due after ten years	18,724
Amortized Cost, Structured Securities	12,121
Amortized Cost, Subtotal	49,312	$ 50,154
Estimated Fair Value, Due in one year or less	1,805
Estimated Fair Value, Due after one year through five years	8,460
Estimated Fair Value, Due after five years through ten years	8,684
Estimated Fair Value, Due after ten years	20,622
Estimated Fair Value, Structured Securities	12,214
Available-for-sale Securities, Debt Securities	$ 51,785	$ 52,409