Investments - Additional Information (Detail) - Maaden Alcoa Joint Venture [Member]	1 Months Ended	3 Months Ended	9 Months Ended
	Dec. 31, 2009	Jun. 30, 2019	Sep. 30, 2019
Schedule Of Equity Method Investments [Line Items]
Non-controlling interest, ownership percentage	25.10%	25.10%	25.10%
Saudi Arabia [Member] | Ma’aden [Member]
Schedule Of Equity Method Investments [Line Items]
Ownership interest in joint venture	74.90%	74.90%	74.90%