Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Allowance for doubtful accounts receivable	$ 1	$ 1
Variable interest entities - assets	239	240
Variable interest entities - liabilities	$ 217	$ 217
Common stock, par value (per share)	$ 0.01
Common stock, authorized shares	6,000,000,000
Common stock, issued shares	201,168,293	214,873,778
Common stock, outstanding shares	201,095,915	214,845,244