Investments (Fair Values by Contractual Maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Maturing in one year or less	$ 17,943	$ 6,733
Maturing after one year through three years	2,947	12,820
Maturing after three years through five years	5,585	13,836
Maturing after five years through ten years	1,008	3,263
Maturing after ten years	0	52
Mortgage- and asset-backed securities	1,935	4,465
Total available-for-sale investments	$ 29,418	$ 41,169