Separation Transaction and Discontinued Operations - Additional Information (Details)					1 Months Ended		2 Months Ended	3 Months Ended		6 Months Ended	12 Months Ended
	Oct. 02, 2017 shares	May 04, 2017	Nov. 01, 2016 USD ($) agreement	Oct. 20, 2016 shares	Feb. 28, 2017 USD ($) $ / shares shares	Sep. 30, 2014 shares	May 31, 2017 USD ($) investment_bank $ / shares shares	Jun. 30, 2017 USD ($) $ / shares shares	Mar. 31, 2017 USD ($)	Jun. 30, 2017 USD ($) $ / shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Investment in common stock of Alcoa Corporation												$ 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		$ 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											(1,000,000)	243,000,000	$ (44,000,000)
Debt issuance and capital expenditures incurred													110,000,000
Impairment charges			$ 0								$ 13,000,000	58,000,000	80,000,000
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
Costs related to separation transaction, after-tax												$ 12,000,000	$ 158,000,000
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%
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
Discontinued operation conversion rate				0.333
Percentage ownership in Alcoa after sale		0.00%
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