Shareholders' equity - Narratives (Details)	12 Months Ended
	Dec. 31, 2017 vote / shares $ / shares shares	Dec. 31, 2016 $ / shares shares
Class of Stock
Authorized preferred shares	30,000,000	30,000,000
Preferred shares, par value (in dollars per share) | $ / shares	$ 0.01	$ 0.01
Class A common shares
Class of Stock
Common stock shares authorized	240,000,000	240,000,000
Voting right per share | vote / shares	0.1
Class B common shares
Class of Stock
Common stock shares authorized	120,000,000	120,000,000
Voting right per share | vote / shares	1
Reduction due to class B common shares owned by a subsidiary (in shares)	18,044,478	18,044,478
Preferred shares at par value
Class of Stock
Authorized preferred shares	30,000,000
Preferred shares, par value (in dollars per share) | $ / shares	$ 0.01
Series A junior participating preferred shares
Class of Stock
Preferred stock reserved for issuance	500,000