Share capital - Share repurchases (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	May 04, 2016
Share repurchase [Abstract]
Stock Repurchase Program, Authorized Amount		$ 100,000,000.0
Stock Repurchased and Retired During Period, Shares	644,768
Payments for Repurchase of Common Stock	$ 7,400,000
Treasury Stock Acquired, Average Cost Per Share	$ 11.46
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 92,600,000