Earnings per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share, Basic and Diluted [Abstract]
Weighted average common shares outstanding during the period - shares used for basic earnings per common share	10,062	10,004	10,037	9,967
Common shares issuable under share based payment plans which are potentially dilutive	73	62	73	68
Common shares used for diluted earnings per common share	10,135	10,066	10,110	10,035
Net income attributable to the Company	$ 2,224	$ 25,391	$ 123,766	$ 28,547
Earnings per common share:
Basic	$ 0.22	$ 2.54	$ 12.33	$ 2.86
Diluted	$ 0.22	$ 2.52	$ 12.24	$ 2.84