Segment Information (Schedule of Revenue and (Loss) Income Before Taxes by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting [Line Items]
Revenues	$ 260,178	$ 199,427
Costs of services	36,657	24,340
Cost of inventory sold	75,791	63,401
SG&A expenses	97,470	70,575
Acquisition-related costs	1,633	8,627
D&A expenses	16,191	10,338
Gain on disposition of property, plant and equipment ("PPE")	(345)	(721)
Foreign exchange gain	(92)	(730)
Operating income	32,873	23,597
Interest expense	(11,310)	(8,133)
Other income, net	913	2,284
Income tax expense	(5,269)	(7,315)
Net income	17,207	10,433
Operating Segments [Member]
Segment Reporting [Line Items]
Revenues	260,178	199,427
Costs of services	36,657	24,340
Cost of inventory sold	75,791	63,401
SG&A expenses	97,470	70,575
Operating income	50,260	41,111
Segment Reconciling Items [Member]
Segment Reporting [Line Items]
Acquisition-related costs	1,633	8,627
D&A expenses	16,191	(10,338)
Gain on disposition of property, plant and equipment ("PPE")	(345)	(721)
Foreign exchange gain	(92)	(730)
Auctions and Marketplaces [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenues	232,567	179,078
Costs of services	21,448	12,587
Cost of inventory sold	75,791	63,401
SG&A expenses	93,002	67,111
Operating income	42,326	35,979
Other Reporting Unit [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenues	27,611	20,349
Costs of services	15,209	11,753
SG&A expenses	4,468	3,464
Operating income	7,934	5,132
Service Revenues [Member]
Segment Reporting [Line Items]
Revenues	176,016	123,379
Service Revenues [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenues	176,016	123,379
Service Revenues [Member] | Auctions and Marketplaces [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenues	148,405	103,030
Service Revenues [Member] | Other Reporting Unit [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenues	27,611	20,349
Revenue from Inventory Sales [Member]
Segment Reporting [Line Items]
Revenues	84,162	76,048
Revenue from Inventory Sales [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenues	84,162	76,048
Revenue from Inventory Sales [Member] | Auctions and Marketplaces [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenues	$ 84,162	$ 76,048