Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Accounts receivable, allowance for doubtful accounts	$ 13	$ 10
Other intangible assets, accumulated amortization	454	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)	816,331,182	816,312,913
Common stock, shares outstanding (in shares)	661,364,449	660,388,128
Treasury stock, shares (in shares)	154,966,733	155,924,785
Variable Interest Entity [Line Items]
Non-recourse debt - current, balance at variable interest entities	$ 2,025	$ 2,164
Consolidated Variable Interest Entities [Member]
Variable Interest Entity [Line Items]
Non-recourse debt - current, balance at variable interest entities	986	1,012
Non-recourse debt - noncurrent, balance at variable interest entities	$ 1,570	$ 1,358