Business Combinations (Schedule of Assets Acquired and Liabilities Assumed) (Details) - USD ($) $ in Thousands		May 31, 2017	Sep. 30, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill acquired on acquisition			$ 673,191	$ 670,922
Iron Planet Holdings Inc. [Member]
Business Acquisition [Line Items]
Purchase price		$ 776,474
Cash and cash equivalents		95,626
Restricted cash		3,000
Trade and other receivables		13,021
Inventory		600
Advances against auction contracts		4,623
Prepaid expenses and deposits		1,645
Income taxes receivable		55
Property, plant and equipment		2,381
Other non-current assets		2,551
Deferred tax assets		1,497
Intangible assets	[1]	188,000
Auction proceeds payable		63,616
Trade and other payables		15,540
Deferred tax liabilities		25,935
Fair value of identifiable net assets acquired		207,908
Goodwill acquired on acquisition		$ 568,566
Iron Planet Holdings Inc. [Member] | Technology [Member]
Business Acquisition [Line Items]
Amortization life		7 years
Minimum [Member] | Iron Planet Holdings Inc. [Member] | Customer Relationships [Member]
Business Acquisition [Line Items]
Amortization life		6 years
Maximum [Member] | Iron Planet Holdings Inc. [Member] | Customer Relationships [Member]
Business Acquisition [Line Items]
Amortization life		13 years

[1] Intangible assets consist of indefinite-lived trade names and trademarks, customer relationships with estimated useful lives of ranging from six to 13 years, and a technology platform with an estimated useful life of seven years.