Restructuring and Other Charges, Net - Additional Information (Detail) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($) Employee	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges	$ 2	$ 113
Curtailment of certain pension benefits	3
Restructuring reserve balance	100		$ 137	$ 47
Payments for restructuring	37		118
Noncurrent portion of the reserve	7		13
Alcoa Corporation [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges		113
PARTER Capital Group AG [Member]
Restructuring Cost and Reserve [Line Items]
Noncurrent portion of the reserve	6		12
Exit Cost [Member] | Alcoa Corporation [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges		103
Closure Cost [Member] | Alcoa Corporation [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges		7
Other Items Charges [Member] | Alcoa Corporation [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges		3
Severance and Other Employee Termination Costs [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring reserve balance	17		35	$ 5
Payments for restructuring	17		19
Cost of Goods Sold [Member] | Alcoa Corporation [Member]
Restructuring Cost and Reserve [Line Items]
Write down of remaining inventories		15
Restructuring And Other Charges [Member] | Alcoa Corporation [Member]
Restructuring Cost and Reserve [Line Items]
Asset impairment		80
Contract termination costs		8
Employee related expenses excluding severance costs		15
Restructuring And Other Charges [Member] | Selling, General Administrative, and Other Expenses [Member] | Alcoa Corporation [Member]
Restructuring Cost and Reserve [Line Items]
Miscellaneous charges		$ 2
2019 Restructuring Plan Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges	$ 1
Number of separated employees associated with employee termination and severance costs | Employee	210
Number of affected employees associated with employee termination and severance costs | Employee	260
Payments for restructuring	$ 13
Reduction in foreign currency	$ 2
Restructuring and related cost number of eliminated positions were not replaced | Employee	60
2019 Restructuring Plan Action [Member] | Severance and Other Employee Termination Costs [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring reserve balance			27
The Point Comfort alumina refinery {Member] | Texas [Member]
Restructuring Cost and Reserve [Line Items]
Number of separated employees associated with employee termination and severance costs | Employee	20
Number of affected employees associated with employee termination and severance costs | Employee	40
Payments for restructuring	$ 1
The Point Comfort alumina refinery {Member] | Severance and Other Employee Termination Costs [Member] | Texas [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring reserve balance			4
Aviles and La Coruna Smelters [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring reserve balance	56		$ 68
Payments for restructuring	$ 12