Separation Transaction and Discontinued Operations - Additional Information (Detail)				1 Months Ended		2 Months Ended	3 Months Ended		12 Months Ended
	Oct. 02, 2017 shares	Nov. 01, 2016 USD ($) agreement	Oct. 20, 2016 shares	Feb. 28, 2017 USD ($) shares	Sep. 30, 2014 shares	May 31, 2017 USD ($) investment_bank $ / shares shares	Jun. 30, 2017 USD ($) $ / shares shares	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Feb. 14, 2017 $ / shares
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Investment in common stock of Alcoa Corporation									$ 0	$ 1,020,000,000
Common stock sold (in shares) | shares	24,022				25,000,000		12,958,767
Common stock sold per share (usd per share) | $ / shares							$ 35.91
Pre-tax gain on sale of common stock							$ 167,000,000		167,000,000
Proceeds from Alcoa Corporation's sale of Yadkin Hydroelectric Project									245,000,000	16,000,000	$ 25,000,000
Debt issuance and capital expenditures incurred										110,000,000
Impairment charges		$ 0							58,000,000	80,000,000	0
Selling, General Administrative, and Other Expenses
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Costs related to separation transaction, pre-tax									18,000,000	193,000,000	24,000,000
Costs related to separation transaction, after-tax									12,000,000	$ 158,000,000	$ 24,000,000
Alcoa Corporation
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Common stock outstanding percentage		80.10%
Percentage of common stock retained			19.90%
Common stock conversion ratio			0.333
Number of common stock retained (in shares) | shares			36,311,767
Common stock sold (in shares) | shares				23,353,000
Common stock sold per share (usd per share) | $ / shares												$ 38.03
Cash proceeds sale of common stock								$ 888,000,000
Pre-tax gain on sale of common stock				$ 351,000,000
Number of counterparties | investment_bank						2
Payment guarantees related to disposal group | agreement		2
Proceeds from Alcoa Corporation's sale of Yadkin Hydroelectric Project							$ 5,000,000	$ 238,000,000	$ 243,000,000
Investment Banks
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Common stock sold (in shares) | shares						12,958,767
Common stock sold per share (usd per share) | $ / shares						$ 35.91
Pre-tax gain on sale of common stock						$ 167,000,000