Segmented information (Schedule of Revenue and Net income by Segment) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting [Line Items]
Revenue	$ 389,050,000	$ 393,222,000	$ 662,305,000	$ 696,651,000
Selling, general and administrative expenses ("SG&A")	100,632,000	97,714,000	199,017,000	192,898,000
Acquisition-related costs		38,000		707,000
Depreciation and amortization expenses ("D&A")	17,857,000	17,112,000	37,150,000	34,227,000
Gain on disposition of property, plant and equipment ("PPE")	(1,213,000)	(101,000)	(1,260,000)	(250,000)
Foreign exchange loss	392,000	403,000	994,000	881,000
Operating income	88,800,000	77,970,000	122,882,000	111,558,000
Interest expense	(8,882,000)	(10,117,000)	(18,064,000)	(20,933,000)
Other income, net	857,000	1,679,000	4,434,000	3,718,000
Income tax expense	(27,656,000)	(15,401,000)	(33,304,000)	(22,040,000)
Net income	53,119,000	54,131,000	75,948,000	72,303,000
Operating Segments [Member]
Segment Reporting [Line Items]
Revenue	389,050,000	393,222,000	662,305,000	696,651,000
Selling, general and administrative expenses ("SG&A")	100,632,000	97,714,000	199,017,000	192,898,000
Segment profit	105,836,000	95,422,000	159,766,000	147,123,000
Acquisition-related costs		38,000		707,000
Depreciation and amortization expenses ("D&A")	17,857,000	17,112,000	37,150,000	34,227,000
Gain on disposition of property, plant and equipment ("PPE")	(1,213,000)	(101,000)	(1,260,000)	(250,000)
Foreign exchange loss	392,000	403,000	994,000	881,000
Operating income	88,800,000	77,970,000	122,882,000	111,558,000
Interest expense	(8,882,000)	(10,117,000)	(18,064,000)	(20,933,000)
Other income, net	857,000	1,679,000	4,434,000	3,718,000
Income tax expense	(27,656,000)	(15,401,000)	(33,304,000)	(22,040,000)
Net income	53,119,000	54,131,000	75,948,000	72,303,000
Auctions and Marketplaces [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue	354,559,000	359,666,000	599,434,000	634,160,000
Selling, general and administrative expenses ("SG&A")	94,559,000	91,466,000	186,144,000	180,648,000
Segment profit	94,676,000	85,831,000	141,286,000	129,851,000
Other Reporting Unit [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue	34,491,000	33,556,000	62,871,000	62,491,000
Selling, general and administrative expenses ("SG&A")	6,073,000	6,248,000	12,873,000	12,250,000
Segment profit	11,160,000	9,591,000	18,480,000	17,272,000
Service Revenue [Member]
Segment Reporting [Line Items]
Revenue	234,139,000	234,606,000	417,262,000	406,978,000
Direct expenses	39,448,000	50,268,000	78,803,000	86,337,000
Service Revenue [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue	234,139,000	234,606,000	417,262,000	406,978,000
Direct expenses	39,448,000	50,268,000	78,803,000	86,337,000
Service Revenue [Member] | Auctions and Marketplaces [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue	199,648,000	201,050,000	354,391,000	344,487,000
Direct expenses	22,190,000	32,551,000	47,285,000	53,368,000
Service Revenue [Member] | Other Reporting Unit [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue	34,491,000	33,556,000	62,871,000	62,491,000
Direct expenses	17,258,000	17,717,000	31,518,000	32,969,000
Inventory Sales Revenue [Member]
Segment Reporting [Line Items]
Revenue	154,911,000	158,616,000	245,043,000	289,673,000
Direct expenses	143,134,000	149,818,000	224,719,000	270,293,000
Inventory Sales Revenue [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue	154,911,000	158,616,000	245,043,000	289,673,000
Direct expenses	143,134,000	149,818,000	224,719,000	270,293,000
Inventory Sales Revenue [Member] | Auctions and Marketplaces [Member] | Operating Segments [Member]
Segment Reporting [Line Items]
Revenue	154,911,000	158,616,000	245,043,000	289,673,000
Direct expenses	$ 143,134,000	$ 149,818,000	$ 224,719,000	$ 270,293,000