Business Combinations (Narrative) (Details) CAD in Thousands, $ in Thousands							1 Months Ended	3 Months Ended		6 Months Ended			12 Months Ended
	May 31, 2017 USD ($)	Nov. 15, 2016 CAD	Nov. 15, 2016 USD ($)	Aug. 01, 2016 USD ($)	Feb. 19, 2016 EUR (€)	Feb. 19, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 EUR (€)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2017 CAD	Jun. 30, 2017 EUR (€)	Jun. 30, 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																$ 667,573			$ 97,537
Acquisition-related costs								$ 4,752			$ 4,752
Net income								17,635	$ 39,710		28,012	$ 69,116
Revenues								166,186	158,805		290,685	290,750
Contributed Revenue								185,485	187,089		340,029	349,893
Contributed Earnings								25,775	28,403		43,825	28,373
Iron Planet Holdings Inc. [Member]
Business Acquisition [Line Items]
Cash consideration	$ 772,706
Total consideration	776,562
Noncash consideration	2,330
Assumption of outstanding options	1,859
Cash related to customary adjustments	333
Goodwill	567,410
Pre-combination attribution	51,678
Post-combination attribution	10,154
Acquisition-related costs	4,752							4,752			4,752
Unvested options recognized as compensation expense	$ 5,402
Contributed Revenue since acquisition date							$ 10,880
Contributed Earnings since acquisition date							$ 292
Acquisition-related costs																29,878
Nonrecurring charges													$ 54,360
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										€ 3,198,000	3,563					$ 3,563
Payment for contingent consideration										€ 1,215,000	1,302
Contingent consideration fair value															1,608,000	1,837		€ 3,080,000	3,431
Goodwill																					$ 19,664
Continuing employment costs										393,000	419
Acquisition-related costs									$ 1,189			$ 1,189				300
Obligation to pay additional amount															€ 1,625,000	1,849
Change in fair value of contingent consideration										€ 178,000	$ 193
Period to make additional amount of obligation upon achievement of certain condition										3 years	3 years
Petrowsky Auctioneers Inc. [Member]
Business Acquisition [Line Items]
Cash consideration				$ 6,250
Total consideration				7,683
Retention payment				750
Contingent consideration fair value																			1,433
Goodwill				4,308
Acquisition-related costs																424
Change in fair value of contingent consideration								$ 1,050			$ 1,457
Period to make additional amount of obligation upon achievement of certain condition										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		CAD 2,500																		$ 1,856
Contingent consideration fair value														CAD 769		593	CAD 725		$ 538
Goodwill																				$ 6,822
Acquisition-related costs																307
Obligation to pay additional amount														CAD 1,000		$ 743
Period to make additional amount of obligation upon achievement of certain condition										3 years	3 years
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
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
Trade Names [Member] | Mascus International Holdings BV [Member]
Business Acquisition [Line Items]
Useful life										5 years	5 years
Trade Names [Member] | Kramer Auctions Ltd. [Member]
Business Acquisition [Line Items]
Useful life										3 years	3 years
Software Assets [Member] | Mascus International Holdings BV [Member]
Business Acquisition [Line Items]
Useful life										5 years	5 years