Stockholders' Equity (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017
Equity, Class of Treasury Stock [Line Items]
Shares repurchased		$ 349	$ 9
Share Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Shares repurchased (in shares)	16
Shares repurchased	$ 338
Stock repurchase program, remaining authorized repurchase amount	$ 1,200	$ 1,200