Mandatory convertible preferred stock dividends.

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The after-tax amount of any dividends on convertible preferred stock.

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of convertible debt securities using the if-converted method.

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Nonvested equity-based payment awards that contain forfeitable rights to dividends or dividend equivalents (whether paid or unpaid) that result in incremental common shares.

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

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The after-tax amount of interest recognized in the period associated with any convertible debt.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

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Net Income or Loss Available to Common Stockholders plus adjustments resulting from the assumption that dilutive convertible securities were converted, options or warrants were exercised, or that other shares were issued upon the satisfaction of certain conditions.

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The aggregate value of preferred stock dividends and other adjustments necessary to derive net income apportioned to common stockholders.

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The earnings that is allocated to common stock and participating securities to the extent that each security may share in earnings as if all of the earnings for the period had been distributed.

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Amount of undistributed earnings (loss) allocated to participating securities for the basic earnings (loss) per share or per unit calculation under the two-class method.

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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Earnings per Share - Reconciliation of Information Used to Compute Basic and Diluted EPS (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Net income (loss) attributable to Alcoa common shareholders	$ 159us-gaap_NetIncomeLoss	$ 149us-gaap_NetIncomeLoss	$ 138us-gaap_NetIncomeLoss	$ (178)us-gaap_NetIncomeLoss	$ (2,339)us-gaap_NetIncomeLoss	$ 24us-gaap_NetIncomeLoss	$ (119)us-gaap_NetIncomeLoss	$ 149us-gaap_NetIncomeLoss	$ 268us-gaap_NetIncomeLoss	$ (2,285)us-gaap_NetIncomeLoss	$ 191us-gaap_NetIncomeLoss
Less: preferred stock dividends declared									21	2	2
Net income (loss) available to common equity									247us-gaap_UndistributedEarnings	(2,287)us-gaap_UndistributedEarnings	189us-gaap_UndistributedEarnings
Less: dividends and undistributed earnings allocated to participating securities									0	0	0
Net income (loss) available to Alcoa common shareholders-basic									247	(2,287)	189
Add: interest expense related to convertible notes									0	0	0
Add: dividends related to mandatory convertible preferred stock									0	0	0
Net income (loss) available to Alcoa common shareholders-diluted									247	(2,287)	189
Average shares outstanding-basic									1,162	1,070	1,067
Stock options									7		3
Stock and performance awards									11		6
Convertible notes									0	0	0
Mandatory convertible preferred stock									$ 0	$ 0	$ 0
Average shares outstanding-diluted									1,180	1,070	1,076