Stockholders' Equity - Additional Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		1 Months Ended			3 Months Ended	12 Months Ended
	Feb. 05, 2018	Dec. 31, 2017	Oct. 31, 2017	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 07, 2018	Sep. 30, 2017
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program amount authorized										$ 2,500
Payment to acquire shares on open market						$ 1,092	$ 38	$ 55
Issuance of common stock during the period				40.5		0.0	41.0	0.0
Price per share of common stock issued during the period				$ 53.23
Proceeds from issuance of common stock				$ 2,160		$ 0	$ 2,188	$ 34
Accelerated Stock Repurchase Program, [Member]
Equity, Class of Treasury Stock [Line Items]
Payment under share repurchase program			$ 1,000
Shares repurchased during period		5.1	15.7
Open-market Stock Purchases [Member]
Equity, Class of Treasury Stock [Line Items]
Shares repurchased during period					1.1
Payment to acquire shares on open market					$ 59
Accelerated Stock Repurchase Program and Open-market Stock Repurchases [Member]
Equity, Class of Treasury Stock [Line Items]
Shares repurchased during period					21.9
Payment to acquire shares on open market					$ 1,100
Price paid per share acquired (in usd per share)					$ 48.33
Subsequent Event [Member]
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program amount authorized									$ 3,000
Subsequent Event [Member] | Accelerated Stock Repurchase Program, [Member]
Equity, Class of Treasury Stock [Line Items]
Payment under share repurchase program	$ 500
Shares repurchased during period	8.5
Price paid per share under ASR (in usd per share)	$ 58.82