Business Combinations (Narrative) (Details) CAD in Thousands, $ in Thousands							3 Months Ended												9 Months Ended	12 Months Ended
	May 31, 2017 USD ($)	Nov. 15, 2016 CAD	Nov. 15, 2016 USD ($)	Aug. 01, 2016 USD ($)	Feb. 19, 2016 EUR (€)	Feb. 19, 2016 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 CAD	Dec. 31, 2017 EUR (€)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 EUR (€)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 CAD	Dec. 31, 2016 EUR (€)	Dec. 31, 2016 USD ($)	Nov. 15, 2016 USD ($)	Feb. 19, 2016 USD ($)
Business Acquisition [Line Items]
Goodwill															$ 91,234									$ 91,234		$ 670,922			$ 97,537
Goodwill impairment																						$ 0	$ 23,574
Impairment loss																						8,911	28,243
Acquisition-related costs																						4,752
Net income (loss)							$ 36,754	$ 10,261	$ 17,635	$ 10,377	$ 27,853	$ (5,137)	$ 39,710	$ 29,406	46,529	$ 20,825	$ 45,083	$ 23,777				75,027	91,832	136,214
Revenues							$ 178,785	141,047	$ 166,186	$ 124,499	$ 146,769	$ 128,876	$ 158,805	$ 131,945	$ 135,462	$ 109,318	$ 155,477	$ 115,618				610,517	566,395	$ 515,875
Contributed Revenue																						659,861	676,234
Contributed Earnings																						94,244	24,179
Iron Planet Holdings Inc. [Member]
Business Acquisition [Line Items]
Cash consideration	$ 772,706
Total consideration	776,474
Noncash consideration	$ 2,330
Voting equity interests acquired, percentage	100.00%
Assumption of outstanding options	$ 1,771
Cash related to customary adjustments	333
Goodwill	568,561
Goodwill remeasurment increase																						1,151
Pre-combination attribution	51,678
Post-combination attribution	10,154
Acquisition-related costs	4,752																					4,752
Unvested options recognized as compensation expense	$ 5,402
Net income (loss)																						12,413
Revenues																						65,641
Acquisition-related costs																										34,653			8,202
Nonrecurring charges																			$ 55,239
Mascus International Holdings BV [Member]
Business Acquisition [Line Items]
Cash consideration					€ 26,553,000	$ 29,580
Total consideration						33,607
Voting equity interests acquired, percentage					100.00%																										100.00%
Contingent consideration					€ 3,198,000																										$ 3,563
Payment for contingent consideration					1,215,000	$ 1,302															€ 1,215,000	1,302
Contingent consideration fair value					€ 3,080,000																				€ 1,879,000	2,255		€ 3,080,000	3,431		3,431
Goodwill																															$ 19,664
Continuing employment costs																					393,000	419
Acquisition-related costs																										552			1,720
Obligation to pay additional amount																									€ 1,625,000	1,849
Change in fair value of contingent consideration																					€ 28,000	$ 33
Period to make additional amount of obligation upon achievement of certain condition																				3 years	3 years	3 years
Petrowsky Auctioneers Inc. [Member]
Business Acquisition [Line Items]
Cash consideration				$ 6,250
Total consideration				7,683
Retention payment				750				$ 333
Contingent consideration fair value																													1,433
Goodwill				4,308
Acquisition-related costs																										653			604
Change in fair value of contingent consideration																						$ 1,457
Period to make additional amount of obligation upon achievement of certain condition																				3 years	3 years	3 years
Petrowsky Auctioneers Inc. [Member] | Maximum [Member]
Business Acquisition [Line Items]
Contingent consideration				3,000
Obligation to pay additional amount				$ 1,000
Kramer Auctions Ltd. [Member]
Business Acquisition [Line Items]
Total cash consideration		CAD 15,300	$ 11,361
Cash consideration		15,000	11,138
Total consideration			11,899
Deferred purchase note consideration		300	$ 223
Contingent consideration		2,500																												$ 1,856
Contingent consideration fair value		CAD 725																								0	CAD 725		538	538
Goodwill																														$ 6,822
Acquisition-related costs																										507			$ 192
Obligation to pay additional amount																				CAD 1,000						$ 743
Additional obligation amount paid																				CAD 333		$ 261
Period to make additional amount of obligation upon achievement of certain condition																				3 years	3 years	3 years
Other income																						$ 620
Customer Relationships [Member] | Iron Planet Holdings Inc. [Member] | Maximum [Member]
Business Acquisition [Line Items]
Useful life	13 years
Customer Relationships [Member] | Mascus International Holdings BV [Member]
Business Acquisition [Line Items]
Useful life																				17 years	17 years	17 years
Customer Relationships [Member] | Petrowsky Auctioneers Inc. [Member]
Business Acquisition [Line Items]
Useful life				10 years
Customer Relationships [Member] | Kramer Auctions Ltd. [Member]
Business Acquisition [Line Items]
Useful life																				10 years	10 years	10 years
Trade Names [Member] | Mascus International Holdings BV [Member]
Business Acquisition [Line Items]
Useful life																				5 years	5 years	5 years
Trade Names [Member] | Kramer Auctions Ltd. [Member]
Business Acquisition [Line Items]
Useful life																				3 years	3 years	3 years
Software Assets [Member] | Mascus International Holdings BV [Member]
Business Acquisition [Line Items]
Useful life																				5 years	5 years	5 years
EquipmentOne [Member]
Business Acquisition [Line Items]
Goodwill impairment																							23,574
EquipmentOne [Member] | Customer Relationships [Member]
Business Acquisition [Line Items]
Impairment loss																							$ 4,669