Investments (Fair Values by Contractual Maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Maturing in one year or less	$ 14,357	$ 17,424
Maturing after one year through three years	4,600	3,356
Maturing after three years through five years	3,987	5,168
Maturing after five years through ten years	859	885
Mortgage- and asset-backed securities	1,849	1,891
Total interest-bearing securities	$ 25,652	$ 28,724