Goodwill and Intangible Assets (Narrative) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($) Segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Goodwill And Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Net	$ 492.1	$ 276.8	$ 282.2
Number of reporting units | Segment	8
Estimated cost of capital, minimum		10.00%
Estimated cost of capital, maximum		13.00%
Estimated cost of capital, weighted		11.00%
Hypothetical decrease to fair values of each reporting unit	10.00%
Net intangible assets	$ 492.1	$ 276.8
Intangibles with indefinite useful lives	70.2
Estimated amortization expense for intangible assets, current year	10.9
Finite-Lived Intangible Assets, Amortization Expense, Year Two	41.2
Finite-Lived Intangible Assets, Amortization Expense, Year Three	37.1
Finite-Lived Intangible Assets, Amortization Expense, Year Four	34.7
Finite-Lived Intangible Assets, Amortization Expense, Year Five	$ 298.0