Investments (Maturities of Fixed Maturity Securities) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 1,561
Amortized Cost, Due after one year through five years	6,889
Amortized Cost, Due after five years through ten years	12,017
Amortized Cost, Due after ten years	24,117
Amortized Cost, Structured Securities	15,642
Amortized Cost	60,226	$ 59,672
Estimated Fair Value, Due in one year or less	1,571
Estimated Fair Value, Due after one year through five years	7,080
Estimated Fair Value, Due after five years through ten years	12,659
Estimated Fair Value, Due after ten years	28,388
Estimated Fair Value, Structured Securities	16,313
Debt Securities, Available-for-sale	$ 66,011	$ 61,348