Operating Expenses (Schedule of Acquisition Related Costs) (Details) - USD ($) $ in Thousands		12 Months Ended
	May 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock option compensation expenses		$ 4,752
Continuing employment costs		35,440	$ 27,856	$ 22,855
Other acquisition-related costs		38,272	11,829	601
Iron Planet Holdings Inc. [Member]
Stock option compensation expenses	$ 4,752	4,752
Legal costs		8,898	3,402
Other acquisition-related costs		21,003	4,800
Mascus International Holdings BV [Member]
Continuing employment costs		530	954
Other acquisition-related costs		22	766
Xcira LLC [Member]
Continuing employment costs		1,811	1,111	191
Other acquisition-related costs				$ 410
Petrowsky Auctioneers Inc. [Member]
Continuing employment costs		649	350
Other acquisition-related costs		4	254
Kramer Auctions Ltd. [Member]
Continuing employment costs		428	76
Other acquisition-related costs		79	$ 116
Other Acquisitions [Member]
Other acquisition-related costs		$ 96