Earnings per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 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	10,033	9,965	10,024	9,948
Common shares issuable under share based payment plans which are potentially dilutive	76	73	75	71
Common shares used for diluted earnings per common share	10,109	10,038	10,099	10,019
Net income attributable to the Company	$ 119,982	$ 1,713	$ 121,542	$ 3,156
Earnings per common share:
Basic	$ 11.96	$ 0.17	$ 12.13	$ 0.32
Diluted	$ 11.87	$ 0.17	$ 12.04	$ 0.32