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Long-term debt - Schedule of Long-term Debt (Parenthetical) (Detail) - Loan Maturity Date Extension [Member] - USD ($)
$ in Millions
 9 Months Ended 12 Months Ended
Sep. 30, 2020
Dec. 31, 2019
Disclosure of detailed information about borrowings [line items]    
Debt, maturity date November 30, 2021  
6.40%, US$4 million, maturing November 30, 2021 [member]    
Disclosure of detailed information about borrowings [line items]    
Debt, interest rate 6.40% 6.40%
Debt $ 4 $ 4
Debt, maturity date November 30, 2021 November 30, 2021
5.85%, US$10 million, maturing November 30, 2021 [member]    
Disclosure of detailed information about borrowings [line items]    
Debt, interest rate 5.85% 5.85%
Debt $ 10 $ 10
Debt, maturity date November 30, 2021 November 30, 2021
4.88%, US$13 million, maturing November 30, 2021 [member]    
Disclosure of detailed information about borrowings [line items]    
Debt, interest rate 4.88% 4.88%
Debt $ 13 $ 13
Debt, maturity date November 30, 2021 November 30, 2021
4.98%, US$6 million, maturing November 30, 2021 [member]    
Disclosure of detailed information about borrowings [line items]    
Debt, interest rate 4.98% 4.98%
Debt $ 6 $ 6
Debt, maturity date November 30, 2021 November 30, 2021
5.23%, US$2 million, maturing November 30, 2021 [member]    
Disclosure of detailed information about borrowings [line items]    
Debt, interest rate 5.23% 5.23%
Debt $ 2 $ 2
Debt, maturity date November 30, 2021 November 30, 2021
4.79%, US$12 million, maturing November 30, 2021 [member]    
Disclosure of detailed information about borrowings [line items]    
Debt, interest rate 4.79% 4.79%
Debt $ 12 $ 12
Debt, maturity date November 30, 2021 November 30, 2021