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Fair Value Measurements - Asset and Liability Measured at Fair Value Measured on Recurring Basis (Detail) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | --- | ---
Assets |  |  |  |
Available for sale customer trust funds assets |  | $ 1,720.1 | $ 1,782.1 | $ 1,755.4
Total assets measured at fair value |  | 1,720.1 | 1,782.1 | 1,755.4
Fair Value, Inputs, Level 2 [Member] |  |  |  |
Assets |  |  |  |
Available for sale customer trust funds assets | [1] | 1,720.1 | 1,782.1 | 1,755.4
Total assets measured at fair value |  | $ 1,720.1 | $ 1,782.1 | $ 1,755.4
[1] | Fair value is based on inputs that are observable for the asset or liability, other than quoted prices.

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References No definition available. + Details
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