Investments (Maturities of Fixed Maturity Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Available-for-sale Securities, Debt Maturities [Abstract]
Amortized Cost, Due in one year or less	$ 1,818
Amortized Cost, Due after one year through five years	7,874
Amortized Cost, Due after five years through ten years	11,672
Amortized Cost, Due after ten years	23,701
Amortized Cost, Subtotal	60,920	$ 60,173
Estimated Fair Value, Due in one year or less	1,817
Estimated Fair Value, Due after one year through five years	7,894
Estimated Fair Value, Due after five years through ten years	11,456
Estimated Fair Value, Due after ten years	25,520
Debt Securities, Available-for-sale	$ 62,608	$ 64,991