Goodwill and Other Intangible Assets - Intangible Assets Subject to Amortization (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 585.4	$ 560.4
Accumulated Amortization	(113.4)	(101.9)
Net Balance	472.0	458.5
Noncompetition agreements
Finite-Lived Intangible Assets [Line Items]
Gross Amount	4.2	6.3
Accumulated Amortization	(4.1)	(6.2)
Net Balance	0.1	0.1
Customer Relationships
Finite-Lived Intangible Assets [Line Items]
Gross Amount	91.3	65.6
Accumulated Amortization	(35.6)	(30.4)
Net Balance	55.7	35.2
Operating Permits
Finite-Lived Intangible Assets [Line Items]
Gross Amount	460.8	459.0
Accumulated Amortization	(48.4)	(42.3)
Net Balance	412.4	416.7
Use Rights And Other
Finite-Lived Intangible Assets [Line Items]
Gross Amount	16.3	16.7
Accumulated Amortization	(13.0)	(12.1)
Net Balance	3.3	4.6
Trade names
Finite-Lived Intangible Assets [Line Items]
Gross Amount	12.8	12.8
Accumulated Amortization	(12.3)	(10.9)
Net Balance	$ 0.5	$ 1.9