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	$ 14,608
Amortized Cost, Due after one year through five years	53,506
Amortized Cost, Due after five years through ten years	58,491
Amortized Cost, Due after ten years	112,601
Amortized Cost, Structured Securities	49,353
Amortized Cost, Subtotal	288,559	$ 286,816
Estimated Fair Value, Due in one year or less	14,625
Estimated Fair Value, Due after one year through five years	55,316
Estimated Fair Value, Due after five years through ten years	61,515
Estimated Fair Value, Due after ten years	126,496
Estimated Fair Value, Structured Securities	50,458
Estimated Fair Value of Fixed Maturity Securities AFS	$ 308,410	$ 298,265