Subsequent events (Details) - $ / shares	Feb. 14, 2019	Dec. 31, 2018	Dec. 31, 2017
Class A common shares
Subsequent Event
Common shares, par value (in dollars per share)		$ 0.01	$ 0.01
Class A common shares | Subsequent Event
Subsequent Event
Common shares, par value (in dollars per share)	$ 0.01
Class B common shares
Subsequent Event
Common shares, par value (in dollars per share)		$ 0.01	$ 0.01
Class B common shares | Subsequent Event
Subsequent Event
Common shares, par value (in dollars per share)	$ 0.01