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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.

X

- 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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- References

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X

- 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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