Investments (Maturities of Fixed Maturity Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities [Abstract]
Amortized Cost, Due in one year or less	$ 12,720
Amortized Cost, Due after one year through five years	63,453
Amortized Cost, Due after five years through ten years	60,957
Amortized Cost, Due after ten years	99,559
Amortized Cost, Structured Securities	49,995
Amortized Cost, Subtotal	286,684	$ 271,701
Estimated Fair Value, Due in one year or less	12,827
Estimated Fair Value, Due after one year through five years	66,568
Estimated Fair Value, Due after five years through ten years	64,549
Estimated Fair Value, Due after ten years	113,649
Estimated Fair Value, Structured Securities	51,301
Available-for-sale Securities, Debt Securities	$ 308,894	$ 289,563