Shareholders' equity - Preferred shares (Details) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Preferred shares, shares authorized (in shares)	30,000,000	30,000,000
Preferred shares, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred shares at par value
Class of Stock [Line Items]
Preferred shares, shares authorized (in shares)	30,000,000
Preferred shares, par value (in dollars per share)	$ 0.01
Series A junior participating preferred shares
Class of Stock [Line Items]
Preferred stock reserved for issuance	500,000