Segment Information (Schedule of Revenue and (Loss) Income Before Taxes by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting [Line Items]
Revenues	$ 245,346	$ 227,401	$ 814,054	$ 679,441
Costs of services	33,053	33,461	112,743	94,093
Cost of inventory sold	74,341	72,476	231,834	207,603
SG&A expenses	88,323	85,335	287,052	230,287
Impairment loss				8,911
Acquisition-related costs	2,007	3,587	5,039	35,162
Depreciation and amortization expenses ("D&A")	16,723	14,837	49,451	37,047
Gain on disposition of property, plant and equipment ("PPE")	(342)	(42)	(958)	(1,071)
Foreign exchange loss (gain)	47	816	31	(7)
Operating income	31,194	16,931	128,862	67,416
Interest expense	(10,473)	(10,558)	(32,720)	(27,311)
Other income, net	7,182	592	8,995	6,346
Income tax expense	(4,791)	3,358	(19,091)	(7,982)
Net income	23,112	10,323	86,046	38,469
Operating Segments [Member]
Segment Reporting [Line Items]
Revenues	245,346	227,401	814,054	679,441
Costs of services	33,053	33,461	112,743	94,093
Cost of inventory sold	74,341	72,476	231,834	207,603
SG&A expenses	88,323	85,335	287,052	230,287
Impairment loss				8,911
Operating income	49,629	36,129	182,425	138,547
Segment Reconciling Items [Member]
Segment Reporting [Line Items]
Acquisition-related costs	2,007	3,587	5,039	35,162
Depreciation and amortization expenses ("D&A")	16,723	(14,837)	49,451	(37,047)
Gain on disposition of property, plant and equipment ("PPE")	(342)	(42)	(958)	(1,071)
Foreign exchange loss (gain)	47	816	31	(7)
Auctions and Marketplaces [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenues	218,576	202,718	725,394	608,169
Costs of services	20,059	18,381	62,888	51,946
Cost of inventory sold	74,341	72,476	231,834	207,603
SG&A expenses	83,542	81,736	272,503	219,824
Impairment loss				8,911
Operating income	40,634	30,125	158,169	119,885
Other Reporting Unit [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenues	26,770	24,683	88,660	71,272
Costs of services	12,994	15,080	49,855	42,147
SG&A expenses	4,781	3,599	14,549	10,463
Operating income	8,995	6,004	24,256	18,662
Service Revenues [Member]
Segment Reporting [Line Items]
Revenues	161,374	145,938	551,736	442,066
Service Revenues [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenues	161,374	145,938	551,736	442,066
Service Revenues [Member] | Auctions and Marketplaces [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenues	134,604	121,255	463,076	370,794
Service Revenues [Member] | Other Reporting Unit [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenues	26,770	24,683	88,660	71,272
Revenue from Inventory Sales [Member]
Segment Reporting [Line Items]
Revenues	83,972	81,463	262,318	237,375
Revenue from Inventory Sales [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenues	83,972	81,463	262,318	237,375
Revenue from Inventory Sales [Member] | Auctions and Marketplaces [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenues	$ 83,972	$ 81,463	$ 262,318	$ 237,375