Common Stock (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Nov. 30, 2016
Equity, Class of Treasury Stock [Line Items]
Payments for repurchase of common stock	$ 250	$ 200
Stock-based compensation, shares issued during period	2.3	3.3
Common Stock
Equity, Class of Treasury Stock [Line Items]
Common stock repurchased (in shares)	3.0	2.7
Payments for repurchase of common stock	$ 250	$ 200
Share repurchases program, authorized amount (up to)			$ 2,500
Stock repurchase program, remaining authorized repurchase amount	$ 1,300