Earnings per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share, Basic and Diluted [Abstract]
Weighted average common shares outstanding during the period - shares used for basic earnings per common share	10,004	9,865	9,967	9,860
Common shares issuable under share based payment plans which are potentially dilutive	62	43	68	42
Common shares used for diluted earnings per common share	10,066	9,908	10,035	9,902
Net income attributable to the Company	$ 25,391	$ 1,957	$ 28,547	$ 4,551
Earnings per common share:
Basic	$ 2.54	$ .20	$ 2.86	$ .46
Diluted	$ 2.52	$ .20	$ 2.84	$ .46