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Business Segment, Geographic Area, Product and Significant Customer Information - Summary of Goodwill Associated with Reportable Segments (Detail) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | --- | ---
Segment Reporting Information [Line Items] |  |  |
Goodwill | $ 587,861 | $ 591,047 | $ 588,585
Vacuum & Analysis [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Goodwill | 197,193 | 197,617 |
Light & Motion [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Goodwill | $ 390,668 | $ 393,430 |

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- Definition Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Details
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- Details
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