Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Accounts receivable, allowance for doubtful accounts	$ 16	$ 10
Other intangible assets, accumulated amortization	472	441
Service Concession Asset, Accumulated Depreciation	$ 0	$ 206
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	1,200,000,000	1,200,000,000
Common stock, shares issued (in shares)	817,203,691	816,312,913
Common stock, shares outstanding (in shares)	662,297,479	660,388,128
Treasury stock, shares (in shares)	154,906,212	155,924,785
Variable Interest Entity [Line Items]
Non-recourse debt - current, balance at variable interest entities	$ 1,308	$ 2,164
Consolidated Variable Interest Entities [Member]
Variable Interest Entity [Line Items]
Non-recourse debt - current, balance at variable interest entities	368	1,012
Non-recourse debt - noncurrent, balance at variable interest entities	$ 2,832	$ 1,358