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Business Segment, Geographic Area, and Significant Customer Information - Summary of Goodwill Associated with Reportable Segments (Detail) - USD ($) $ in Millions | Sep. 30, 2021 | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | --- | ---
Segment Reporting Information [Line Items] |  |  |
Goodwill | $ 1,226.2 | $ 1,066.4 | $ 1,058.5
Vacuum & Analysis [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Goodwill | 195.2 | 196.2 |
Light & Motion [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Goodwill | 556.3 | 395.3 |
Equipment & Solutions [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Goodwill | $ 474.7 | $ 474.9 |

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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://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(10)(1)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 2: 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=120320667&loc=SL49117168-202975 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 852 -SubTopic 10 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=84165509&loc=d3e56426-112766 Reference 4: 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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