Segmented information (Schedule of Revenue and Net income by Segment) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting [Line Items]
Revenue	$ 331,542,000	$ 289,796,000	$ 993,847,000	$ 986,447,000
Selling, general and administrative expenses ("SG&A")	110,186,000	93,691,000	309,203,000	286,589,000
Acquisition-related costs		45,000		752,000
Depreciation and amortization expenses ("D&A")	18,436,000	17,692,000	55,586,000	51,919,000
Gain on disposition of property, plant and equipment ("PPE")	(276,000)	(821,000)	(1,536,000)	(1,071,000)
Foreign exchange loss	336,000	237,000	1,330,000	1,118,000
Operating income	67,384,000	40,160,000	190,266,000	151,718,000
Interest expense	(8,737,000)	(10,090,000)	(26,801,000)	(31,023,000)
Other income, net	2,280,000	1,962,000	6,714,000	5,680,000
Income tax expense	(15,437,000)	(6,760,000)	(48,741,000)	(28,800,000)
Net income	45,490,000	25,272,000	121,438,000	97,575,000
Operating Segments [Member]
Segment Reporting [Line Items]
Revenue	331,542,000	289,796,000	993,847,000	986,447,000
Selling, general and administrative expenses ("SG&A")	110,186,000	93,691,000	309,203,000	286,589,000
Segment profit	85,880,000	57,313,000	245,646,000	204,436,000
Acquisition-related costs		45,000		752,000
Depreciation and amortization expenses ("D&A")	18,436,000	17,692,000	55,586,000	51,919,000
Gain on disposition of property, plant and equipment ("PPE")	(276,000)	(821,000)	(1,536,000)	(1,071,000)
Foreign exchange loss	336,000	237,000	1,330,000	1,118,000
Operating income	67,384,000	40,160,000	190,266,000	151,718,000
Interest expense	(8,737,000)	(10,090,000)	(26,801,000)	(31,023,000)
Other income, net	2,280,000	1,962,000	6,714,000	5,680,000
Income tax expense	(15,437,000)	(6,760,000)	(48,741,000)	(28,800,000)
Net income	45,490,000	25,272,000	121,438,000	97,575,000
Auctions and Marketplaces [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue	297,812,000	261,312,000	897,246,000	895,472,000
Selling, general and administrative expenses ("SG&A")	103,933,000	88,138,000	290,077,000	268,786,000
Segment profit	75,893,000	49,333,000	217,179,000	179,184,000
Other Reporting Unit [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue	33,730,000	28,484,000	96,601,000	90,975,000
Selling, general and administrative expenses ("SG&A")	6,253,000	5,553,000	19,126,000	17,803,000
Segment profit	9,987,000	7,980,000	28,467,000	25,252,000
Service Revenue [Member]
Segment Reporting [Line Items]
Revenue	222,679,000	178,577,000	639,941,000	585,555,000
Direct expenses	39,223,000	36,382,000	118,026,000	122,719,000
Service Revenue [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue	222,679,000	178,577,000	639,941,000	585,555,000
Direct expenses	39,223,000	36,382,000	118,026,000	122,719,000
Service Revenue [Member] | Auctions and Marketplaces [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue	188,949,000	150,093,000	543,340,000	494,580,000
Direct expenses	21,733,000	21,431,000	69,018,000	74,799,000
Service Revenue [Member] | Other Reporting Unit [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue	33,730,000	28,484,000	96,601,000	90,975,000
Direct expenses	17,490,000	14,951,000	49,008,000	47,920,000
Inventory Sales Revenue [Member]
Segment Reporting [Line Items]
Revenue	108,863,000	111,219,000	353,906,000	400,892,000
Direct expenses	96,253,000	102,410,000	320,972,000	372,703,000
Inventory Sales Revenue [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue	108,863,000	111,219,000	353,906,000	400,892,000
Direct expenses	96,253,000	102,410,000	320,972,000	372,703,000
Inventory Sales Revenue [Member] | Auctions and Marketplaces [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue	108,863,000	111,219,000	353,906,000	400,892,000
Direct expenses	$ 96,253,000	$ 102,410,000	$ 320,972,000	$ 372,703,000