Goodwill and Intangible Assets (Narrative) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($) Segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Goodwill And Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Net	$ 284.7	$ 282.2	$ 317.1
Number of reporting units | Segment	7
Estimated cost of capital, minimum	9.00%
Estimated cost of capital, maximum	12.00%
Estimated cost of capital, weighted	10.50%
Hypothetical decrease to fair values of each reporting unit	10.00%
Net intangible assets	$ 284.7	$ 282.2
Intangibles with indefinite useful lives	28.5
Estimated amortization expense for intangible assets, current year	7.8
Finite-Lived Intangible Assets, Amortization Expense, Year Two	29.1
Finite-Lived Intangible Assets, Amortization Expense, Year Three	26.4
Finite-Lived Intangible Assets, Amortization Expense, Year Four	22.0
Finite-Lived Intangible Assets, Amortization Expense, Year Five	$ 170.9