Segmented information (Schedule of Revenue and Net income by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting [Line Items]
Revenue	$ 383,413	$ 331,542	$ 389,050	$ 273,255	$ 332,194	$ 289,796	$ 393,222	$ 303,429	$ 355,972	$ 245,346	$ 308,530	$ 260,178	$ 1,377,260	$ 1,318,641	$ 1,170,026
Selling, general and administrative expenses ("SG&A")													417,523	382,389	382,676
Acquisition-related costs													6,014	777	5,093
Depreciation and amortization expenses ("D&A")													74,921	70,501	66,614
Gain on disposition of property, plant and equipment ("PPE")													(1,559)	(1,107)	(2,731)
Foreign exchange loss													1,612	(2,937)	(212)
Operating income	72,892	67,384	88,802	34,082	71,484	40,160	77,970	33,588	56,327	31,194	64,795	32,873	263,160	223,202	185,189
Interest expense													(35,568)	(41,277)	(44,527)
Other income, net													8,296	8,838	11,850
Income tax expense													(65,530)	(41,623)	(31,006)
Net income	$ 48,919	$ 45,490	$ 53,120	$ 22,829	$ 51,565	$ 25,272	$ 54,131	$ 18,172	$ 35,460	$ 23,112	$ 45,727	$ 17,207	170,358	149,140	121,506
Operating Segments [Member]
Segment Reporting [Line Items]
Revenue													1,377,260	1,318,641	1,170,026
Selling, general and administrative expenses ("SG&A")													417,523	382,389	382,676
Segment profit													344,148	290,436	253,953
Acquisition-related costs														777	5,093
Depreciation and amortization expenses ("D&A")													74,921	70,501	66,614
Gain on disposition of property, plant and equipment ("PPE")													(1,559)	(1,107)	(2,731)
Foreign exchange loss													1,612	(2,937)	(212)
Operating income													263,160	223,202	185,189
Interest expense													(35,568)	(41,277)	(44,527)
Other income, net													8,296	8,838	11,850
Income tax expense													(65,530)	(41,623)	(31,006)
Net income													170,358	149,140	121,506
Auctions and Marketplaces [Member]
Segment Reporting [Line Items]
Segment profit														254,764
Auctions and Marketplaces [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue													1,245,707	1,193,440	1,046,518
Selling, general and administrative expenses ("SG&A")													388,442	358,016	363,549
Segment profit													306,777		221,200
Other Reporting Unit [Member]
Segment Reporting [Line Items]
Segment profit														35,672
Other Reporting Unit [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue													131,553	125,201	123,508
Selling, general and administrative expenses ("SG&A")													29,081	24,373	19,127
Segment profit													37,371		32,753
Service Revenue [Member]
Segment Reporting [Line Items]
Revenue													871,596	804,024	749,515
Direct expenses													157,296	164,977	159,058
Service Revenue [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue													871,596	804,024	749,515
Direct expenses													157,296	164,977	159,058
Service Revenue [Member] | Auctions and Marketplaces [Member]
Segment Reporting [Line Items]
Revenue															626,007
Service Revenue [Member] | Auctions and Marketplaces [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue													740,043	678,823
Direct expenses													92,195	99,821	87,430
Service Revenue [Member] | Other Reporting Unit [Member]
Segment Reporting [Line Items]
Revenue															123,508
Service Revenue [Member] | Other Reporting Unit [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue													131,553	125,201
Direct expenses													65,101	65,156	71,628
Inventory Sales Revenue [Member]
Segment Reporting [Line Items]
Revenue													505,664	514,617	420,511
Direct expenses													458,293	480,839	374,339
Inventory Sales Revenue [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue													505,664	514,617	420,511
Direct expenses													458,293	480,839	374,339
Inventory Sales Revenue [Member] | Auctions and Marketplaces [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue													505,664	514,617	420,511
Direct expenses													$ 458,293	$ 480,839	$ 374,339