Business Combinations (Narrative) (Details) - USD ($) $ in Thousands		3 Months Ended
	May 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill		$ 674,097		$ 670,922
Acquisition-related costs		195
Net income (loss)		17,138	$ 10,377
Iron Planet Holdings Inc. [Member]
Business Acquisition [Line Items]
Cash consideration	$ 772,706
Total consideration	776,474
Cash consideration, placed in escrow	35,000
Noncash consideration	2,330
Non-cash consideration, placed in escrow	$ 1,771
Amount released from escrow		36,771
Voting equity interests acquired, percentage	100.00%
Cash related to customary adjustments	$ 333
Goodwill	$ 568,137
Acquisition-related costs			$ 7,691
Acquisition-related costs		$ 639