Investments - Schedule of Equity Investment (Detail)	12 Months Ended
	Dec. 31, 2019
Maaden Aluminium Compay [Member] | Saudi Arabia [Member]
Schedule of Equity Method Investments [Line Items]
Nature of investment	Aluminum smelter and casthouse
Percent of equity investments in other entity	25.10%
Maaden Bauxite and Alumina CO [Member] | Saudi Arabia [Member]
Schedule of Equity Method Investments [Line Items]
Nature of investment	Bauxite mine and alumina refinery
Percent of equity investments in other entity	25.10%
Halco Mining Inc [Member] | GUINEA
Schedule of Equity Method Investments [Line Items]
Nature of investment	Bauxite mine
Percent of equity investments in other entity	45.00%
Energética Barra Grande S.A. [Member] | Brazil [Member]
Schedule of Equity Method Investments [Line Items]
Nature of investment	Hydroelectric generation facility
Percent of equity investments in other entity	42.18%
Pechiney Reynolds Quebec Inc [Member] | Canada [Member]
Schedule of Equity Method Investments [Line Items]
Nature of investment	Aluminum smelter
Percent of equity investments in other entity	50.00%
Consorcio Serra Do Facao [Member] | Brazil [Member]
Schedule of Equity Method Investments [Line Items]
Nature of investment	Hydroelectric generation facility
Percent of equity investments in other entity	34.97%
MRN [Member] | Brazil [Member]
Schedule of Equity Method Investments [Line Items]
Nature of investment	Bauxite mine
Percent of equity investments in other entity	18.20%
Manicouagan Power Limited Partnership [Member] | Canada [Member]
Schedule of Equity Method Investments [Line Items]
Nature of investment	Hydroelectric generation facility
Percent of equity investments in other entity	40.00%
Elysis TM Limited Partnership [Member] | Canada [Member]
Schedule of Equity Method Investments [Line Items]
Nature of investment	Aluminum smelting technology
Percent of equity investments in other entity	48.235%