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	$ 1,729
Amortized Cost, Due after one year through five years	8,201
Amortized Cost, Due after five years through ten years	11,036
Amortized Cost, Due after ten years	23,696
Amortized Cost, Structured Securities	14,410
Amortized Cost, Subtotal	59,072	$ 58,599
Estimated Fair Value, Due in one year or less	1,737
Estimated Fair Value, Due after one year through five years	8,254
Estimated Fair Value, Due after five years through ten years	10,927
Estimated Fair Value, Due after ten years	25,379
Estimated Fair Value, Structured Securities	14,330
Debt Securities, Available-for-sale	$ 60,627	$ 63,333