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	$ 1,730
Amortized Cost, Due after one year through five years	8,028
Amortized Cost, Due after five years through ten years	11,339
Amortized Cost, Due after ten years	23,803
Amortized Cost, Structured Securities	13,623
Amortized Cost, Subtotal	58,523	$ 58,599
Estimated Fair Value, Due in one year or less	1,736
Estimated Fair Value, Due after one year through five years	8,111
Estimated Fair Value, Due after five years through ten years	11,234
Estimated Fair Value, Due after ten years	26,036
Estimated Fair Value, Structured Securities	13,636
Debt Securities, Available-for-sale	$ 60,753	$ 63,333