Common Stock (Details) - USD ($) shares in Millions, $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Nov. 30, 2016
Equity, Class of Treasury Stock [Line Items]
Payments for repurchase of common stock	$ 675	$ 600
Stock-based compensation, shares issued during period	3.0	4.6
Common Stock
Equity, Class of Treasury Stock [Line Items]
Common stock repurchased (in shares)	8.2	8.0
Payments for repurchase of common stock	$ 675	$ 600
Share repurchases program, authorized amount (up to)			$ 2,500
Stock repurchase program, remaining authorized repurchase amount	$ 866