Segmented information (Schedule of Revenue and Net income by Segment) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting [Line Items]
Revenue	$ 396,361,000	$ 389,050,000	$ 727,916,000	$ 662,305,000
Selling, general and administrative expenses ("SG&A")	111,819,000	100,632,000	227,897,000	199,017,000
Acquisition-related costs	3,049,000		5,971,000
Depreciation and amortization expenses ("D&A")	21,935,000	17,857,000	43,005,000	37,150,000
Gain on disposition of property, plant and equipment ("PPE")	(175,000)	(1,213,000)	(243,000)	(1,260,000)
Foreign exchange loss	151,000	392,000	428,000	994,000
Operating income	89,517,000	88,800,000	134,019,000	122,882,000
Interest expense	(8,867,000)	(8,882,000)	(17,813,000)	(18,064,000)
Other income, net	1,196,000	857,000	2,198,000	4,434,000
Income tax expense	(21,065,000)	(27,656,000)	(29,484,000)	(33,304,000)
Net income	60,781,000	53,119,000	88,920,000	75,948,000
Operating Segments [Member]
Segment Reporting [Line Items]
Revenue	396,361,000	389,050,000	727,916,000	662,305,000
Selling, general and administrative expenses ("SG&A")	111,819,000	100,632,000	227,897,000	199,017,000
Segment profit	114,477,000	105,836,000	183,180,000	159,766,000
Acquisition-related costs	3,049,000		5,971,000
Depreciation and amortization expenses ("D&A")	21,935,000	17,857,000	43,005,000	37,150,000
Gain on disposition of property, plant and equipment ("PPE")	(175,000)	(1,213,000)	(243,000)	(1,260,000)
Foreign exchange loss	151,000	392,000	428,000	994,000
Operating income	89,517,000	88,800,000	134,019,000	122,882,000
Interest expense	(8,867,000)	(8,882,000)	(17,813,000)	(18,064,000)
Other income, net	1,196,000	857,000	2,198,000	4,434,000
Income tax expense	(21,065,000)	(27,656,000)	(29,484,000)	(33,304,000)
Net income	60,781,000	53,119,000	88,920,000	75,948,000
Auctions and Marketplaces [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue	355,088,000	354,559,000	651,368,000	599,434,000
Selling, general and administrative expenses ("SG&A")	101,417,000	94,559,000	205,762,000	186,144,000
Segment profit	100,663,000	94,676,000	160,261,000	141,286,000
Other Reporting Unit [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue	41,273,000	34,491,000	76,548,000	62,871,000
Selling, general and administrative expenses ("SG&A")	10,402,000	6,073,000	22,135,000	12,873,000
Segment profit	13,814,000	11,160,000	22,919,000	18,480,000
Service Revenue [Member]
Segment Reporting [Line Items]
Revenue	252,748,000	234,139,000	458,778,000	417,262,000
Direct expenses	39,042,000	39,448,000	75,069,000	78,803,000
Service Revenue [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue	252,748,000	234,139,000	458,778,000	417,262,000
Direct expenses	39,042,000	39,448,000	75,069,000	78,803,000
Service Revenue [Member] | Auctions and Marketplaces [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue	211,475,000	199,648,000	382,230,000	354,391,000
Direct expenses	21,985,000	22,190,000	43,575,000	47,285,000
Service Revenue [Member] | Other Reporting Unit [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue	41,273,000	34,491,000	76,548,000	62,871,000
Direct expenses	17,057,000	17,258,000	31,494,000	31,518,000
Inventory Sales Revenue [Member]
Segment Reporting [Line Items]
Revenue	143,613,000	154,911,000	269,138,000	245,043,000
Direct expenses	131,023,000	143,134,000	241,770,000	224,719,000
Inventory Sales Revenue [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue	143,613,000	154,911,000	269,138,000	245,043,000
Direct expenses	131,023,000	143,134,000	241,770,000	224,719,000
Inventory Sales Revenue [Member] | Auctions and Marketplaces [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue	143,613,000	154,911,000	269,138,000	245,043,000
Direct expenses	$ 131,023,000	$ 143,134,000	$ 241,770,000	$ 224,719,000