Common Stock (Details) - USD ($) shares in Millions, $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Nov. 30, 2016
Equity, Class of Treasury Stock [Line Items]
Payments for repurchase of common stock	$ 500	$ 400
Stock-based compensation, shares issued during period	2.5	3.7
Common Stock
Equity, Class of Treasury Stock [Line Items]
Common stock repurchased (in shares)	6.1	5.4
Payments for repurchase of common stock	$ 500	$ 400
Share repurchases program, authorized amount (up to)			$ 2,500
Stock repurchase program, remaining authorized repurchase amount	$ 1,000