Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Allowances for accounts and notes receivable	$ 14	$ 14
Property and equipment, accumulated depreciation and amortization	$ 645	$ 704
Common stock, par value (in usd per share)
Common stock, shares authorized	Unlimited	Unlimited
Common stock, shares issued	253,828,112	251,532,493
Common stock, shares outstanding	253,828,112	251,532,493

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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A valuation allowance relating to a written agreement to receive money (at a specified future date(s) within one year from the reporting date (or the normal operating cycle, whichever is longer), consisting of principal as well as any accrued interest) for the portion that is expected to be uncollectible.

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Face amount per share of no-par value common stock.

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Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.

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Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation.

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