Commitments and contingencies - Narrative (Details) - t t in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Purchase commitment of methanol supply \| Within one year
Disclosure of associates [line items]
Purchase commitments (in tonnes)	1.2
Purchase commitment of methanol supply \| Thereafter
Disclosure of associates [line items]
Purchase commitments (in tonnes)	1.2
Atlas Joint Venture
Disclosure of associates [line items]
Percentage of marketing rights in production	100.00%
Percentage of ownership interest in associates	63.10%	63.10%
Atlas Joint Venture \| Trinidad
Disclosure of associates [line items]
Percentage of ownership interest in associates	63.10%
Atlas Joint Venture \| Egypt
Disclosure of associates [line items]
Percentage of ownership interest in associates	50.00%
Atlas Plant and Egypt Plant \| Purchase commitment of methanol supply
Disclosure of associates [line items]
Purchase commitments (in tonnes)	1.2