Investments (Maturities of Fixed Maturity Securities) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Available-for-sale Securities, Debt Maturities [Abstract]
Amortized Cost, Due in one year or less	$ 1,373
Amortized Cost, Due after one year through five years	7,490
Amortized Cost, Due after five years through ten years	11,861
Amortized Cost, Due after ten years	23,371
Amortized Cost, Structured Securities	15,791
Amortized Cost, Subtotal	59,886	$ 59,672
Estimated Fair Value, Due in one year or less	1,379
Estimated Fair Value, Due after one year through five years	7,613
Estimated Fair Value, Due after five years through ten years	12,112
Estimated Fair Value, Due after ten years	26,359
Estimated Fair Value, Structured Securities	16,127
Debt Securities, Available-for-sale	$ 63,590	$ 61,348