Properties, Plants, and Equipment, Net - Schedule of Properties, Plants, and Equipment, Net (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Less: accumulated depreciation, depletion, and amortization	$ 13,799	$ 13,480
Properties, plants, and equipment excluding construction work-in-progress	7,595	7,921
Construction work-in-progress	321	406
Properties, plants, and equipment, net	7,916	8,327
Land [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	333	328
Structures [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	8,126	8,300
Machinery and Equipment [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	12,935	12,773
Property Plant And Equipment Other Than Construction In Progress [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	21,394	21,401
Operating Segments [Member] | Bauxite Mining [Member] | Structures [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	1,138	1,119
Operating Segments [Member] | Bauxite Mining [Member] | Machinery and Equipment [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	499	480
Operating Segments [Member] | Alumina Refining [Member] | Structures [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	2,415	2,478
Operating Segments [Member] | Alumina Refining [Member] | Machinery and Equipment [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	3,956	3,604
Operating Segments [Member] | Aluminum Smelting and Casting [Member] | Structures [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	3,457	3,532
Operating Segments [Member] | Aluminum Smelting and Casting [Member] | Machinery and Equipment [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	6,251	6,489
Operating Segments [Member] | Energy [Member] | Structures [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	440	506
Operating Segments [Member] | Energy [Member] | Machinery and Equipment [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	879	889
Operating Segments [Member] | Aluminum Rolling [Member] | Structures [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	290	285
Operating Segments [Member] | Aluminum Rolling [Member] | Machinery and Equipment [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	1,057	1,029
Other [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, net	519	520
Other [Member] | Structures [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	386	380
Other [Member] | Machinery and Equipment [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	$ 293	$ 282