Segmented Information (Schedule of Revenue and (Loss) Income Before Taxes by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting [Line Items]
Revenues	$ 303,429	$ 260,178
Direct expenses	36,069	36,657
SG&A expenses	95,184	97,470
Segment profit		50,260
Acquisition-related costs	669	1,633
D&A expenses	17,115	16,191
Gain on disposition of PPE	(149)	(345)
Foreign exchange (gain) loss	478	(92)
Operating income	33,588	32,873
Interest expense	(10,816)	(11,310)
Other income, net	2,039	913
Income tax expense	(6,639)	(5,269)
Net income	18,172	17,207
Operating Segments [Member]
Segment Reporting [Line Items]
Revenues	303,429
SG&A expenses	95,184
Segment profit	51,701
D&A expenses	17,115
Operating income	33,588
Interest expense	(10,816)
Other income, net	2,039
Income tax expense	(6,639)
Net income	18,172
Segment Reconciling Items [Member]
Segment Reporting [Line Items]
Acquisition-related costs	669
Gain on disposition of PPE	(149)
Foreign exchange (gain) loss	478
Auctions and Marketplaces [Member]
Segment Reporting [Line Items]
Revenues		232,567
SG&A expenses		93,002
Segment profit		42,326
Auctions and Marketplaces [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenues	274,494
SG&A expenses	89,182
Segment profit	44,020
Other Reporting Unit [Member]
Segment Reporting [Line Items]
Revenues		27,611
SG&A expenses		4,468
Segment profit		7,934
Other Reporting Unit [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenues	28,935
SG&A expenses	6,002
Segment profit	7,681
Service Revenues [Member]
Segment Reporting [Line Items]
Revenues	172,372	176,016
Direct expenses	36,069	36,657
Service Revenues [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenues	172,372
Direct expenses	36,069
Service Revenues [Member] | Auctions and Marketplaces [Member]
Segment Reporting [Line Items]
Revenues		148,405
Direct expenses		21,448
Service Revenues [Member] | Auctions and Marketplaces [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenues	143,437
Direct expenses	20,817
Service Revenues [Member] | Other Reporting Unit [Member]
Segment Reporting [Line Items]
Revenues		27,611
Direct expenses		15,209
Service Revenues [Member] | Other Reporting Unit [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenues	28,935
Direct expenses	15,252
Inventory Sales Revenue [Member]
Segment Reporting [Line Items]
Revenues	131,057	84,162
Direct expenses	120,475	75,791
Inventory Sales Revenue [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenues	131,057
Direct expenses	120,475
Inventory Sales Revenue [Member] | Auctions and Marketplaces [Member]
Segment Reporting [Line Items]
Revenues		84,162
Direct expenses		$ 75,791
Inventory Sales Revenue [Member] | Auctions and Marketplaces [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenues	131,057
Direct expenses	$ 120,475