Cash Flow Information - Additional Information (Details) $ / shares in Units, $ in Millions		1 Months Ended	3 Months Ended
	Oct. 02, 2017 $ / shares shares	Sep. 30, 2014 shares	Jun. 30, 2017 USD ($) shares
Schedule Of Common Stock [Line Items]
Depository shares sold (in shares)	24,022	25,000,000	12,958,767
Preferred stock conversion rate		0.10
Conversion of stock, shares converted (in shares)	31,420
Alcoa Corporation | Electricity Contract
Schedule Of Common Stock [Line Items]
Outstanding notes held by Investment banks, including accrued and unpaid interest | $			$ 465
Mandatory convertible preferred stock
Schedule Of Common Stock [Line Items]
Depository shares sold (in shares)	24,975,978
Preferred stock conversion rate	0.1
Preferred stock conversion price (in usd per share) | $ / shares	$ 1.56996
Conversion of stock, shares converted (in shares)	39,211,286