Shareholders' equity - Narratives (Details)	12 Months Ended
	Dec. 31, 2018 vote / shares shares	Dec. 31, 2017 shares
Class A common shares
Class of Stock [Line Items]
Common stock shares authorized | shares	240,000,000	240,000,000
Voting right per share | vote / shares	0.1
Class B common shares
Class of Stock [Line Items]
Common stock shares authorized | shares	120,000,000	120,000,000
Voting right per share | vote / shares	1