Operating Expenses (Schedule of Acquisition Related Costs) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Iron Planet Holdings Inc. [Member] | Other Related Costs [Member]
Acquisition-related costs	$ 2,944	$ 34,653	$ 8,202
Other Acquisitions [Member]
Acquisition-related costs	5,093	38,272	11,829
Other Acquisitions [Member] | Employment Costs [Member]
Acquisition-related costs	2,091	3,418	2,491
Other Acquisitions [Member] | Other Related Costs [Member]
Acquisition-related costs	$ 58	$ 201	$ 1,136