Contingencies and Commitments - Additional Information - 2 (Detail) $ in Millions, ر.س in Billions	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 31, 2019 SAR (ر.س)	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)
Other Noncurrent Liabilities and Deferred Credits [Member]
Other Commitments [Line Items]
Guarantee issued on behalf of smelting and rolling mill companies	$ 1			$ 1
Financial Guarantee [Member]
Other Commitments [Line Items]
Debt service requirements, principal	50
Debt service requirements, interest maximum	10
Alcoa Corporation [Member]
Other Commitments [Line Items]
Contribution to joint venture			$ 66
Alcoa Corporation [Member]
Other Commitments [Line Items]
Project financing Investment	$ 1,179
Saudi Arabia [Member] | Ma'aden Joint Venture [Member]
Other Commitments [Line Items]
Contribution to joint venture			$ 199
Ma'aden Joint Venture [Member] | Saudi Arabia [Member]
Other Commitments [Line Items]
Ownership interest in joint venture	74.90%	74.90%
Alcoa Joint Venture [Member]
Other Commitments [Line Items]
Ownership interest in joint venture	25.10%	25.10%
Maaden Alcoa Joint Venture [Member]
Other Commitments [Line Items]
Equity investments	$ 863			$ 874
Maaden Alcoa Joint Venture [Member] | Alcoa Corporation [Member]
Other Commitments [Line Items]
Expected project investment	1,100
Capital investment commitment paid-to-date	982
Maaden Alcoa Joint Venture [Member] | Saudi Arabia [Member]
Other Commitments [Line Items]
Expected project investment	10,800	ر.س 40.5
Alcoa Corporation [Member]
Other Commitments [Line Items]
Project financing Investment	$ 296