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SEGMENT INFORMATION - Narrative (FY) (Details) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 Segment | Dec. 31, 2017 USD ($) Segment | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items] |  |  |  |
Number of reportable segment | Segment | 2 | 2 |  |
United States |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Export sales | $ |  | $ 448.0 | $ 470.5 | $ 508.4

X
- Definition Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements. + References No definition available. + Details
Name: | us-gaap_NumberOfReportableSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- 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
Name: | us-gaap_SegmentReportingInformationLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Goods or services sold to a foreign country. + References No definition available. + Details
Name: | wab_ExportSales
Namespace Prefix: | wab_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | us-gaap_StatementGeographicalAxis=country_US
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |