Operating expenses (Schedule of Acquisition Related Costs) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Acquisition-related costs	$ 6,014	$ 777	$ 5,093
Rouse Services LLC [Member]
Acquisition-related costs	6,014
Rouse Services LLC [Member] | Employment Costs [Member]
Acquisition-related costs	802
Rouse Services LLC [Member] | Other Related Costs [Member]
Acquisition-related costs	$ 5,212
Iron Planet Holdings Inc. [Member] | Other Related Costs [Member]
Acquisition-related costs		82	2,944
Other Acquisitions [Member]
Acquisition-related costs		777	5,093
Other Acquisitions [Member] | Employment Costs [Member]
Acquisition-related costs		128	2,091
Other Acquisitions [Member] | Other Related Costs [Member]
Acquisition-related costs		$ 567	$ 58