Stockholders' Equity (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jul. 31, 2019
Equity, Class of Treasury Stock [Line Items]
Shares repurchased	$ 236	$ 30	$ 2	$ 8
Share Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Shares repurchased (in shares)					16
Shares repurchased					$ 250
Stock repurchase program, remaining authorized repurchase amount	$ 550				$ 550
Subsequent Event | Share Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, remaining authorized repurchase amount						$ 1,500