Shareholders' equity (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Class of Stock [Line Items]
Authorized preferred shares	30,000,000	30,000,000
Preferred shares, par value (in dollars per share)	$ 0.01	$ 0.01
Class A common shares
Class of Stock [Line Items]
Common stock shares authorized	240,000,000	240,000,000
Class B common shares
Class of Stock [Line Items]
Common stock shares authorized	120,000,000	120,000,000
Reduction due to class B common shares owned by a subsidiary, shares	18,044,478	18,044,478
Preferred shares at par value
Class of Stock [Line Items]
Authorized preferred 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