Equity - Equity Repurchase Program, Additional Information (Details) - USD ($) shares in Millions	12 Months Ended
	Dec. 31, 2017	Oct. 31, 2017
Equity, Class of Treasury Stock [Line Items]
Shares repurchased (in shares)	20.2
Shares repurchased	$ 1,028,000,000
October 2016 Share Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Amount authorized under equity repurchase program	1,300,000,000.0	$ 1,300,000,000.0
Prior to October 2016 Share Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Amount authorized under equity repurchase program	$ 4,375,000,000