Goodwill and Other Intangible Assets - Changes in Goodwill (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Goodwill [Line Items]
Balance at beginning of period	$ 2,396.8	$ 2,399.1
Measurement period adjustments		(1.6)
Acquisitions	17.3
Goodwill reclassified from/(allocated to) assets held for sale		(0.2)
Divestitures	(0.1)	(0.5)
Balance at end of period	2,414.0	2,396.8
East Group
Goodwill [Line Items]
Balance at beginning of period	574.3	576.1
Measurement period adjustments		(1.6)
Goodwill reclassified from/(allocated to) assets held for sale	0.1	(0.2)
Transfer of operations from East Group to West Group	(1.9)
Balance at end of period	572.5	574.3
West Group
Goodwill [Line Items]
Balance at beginning of period	1,822.5	1,823.0
Acquisitions	17.3
Goodwill reclassified from/(allocated to) assets held for sale	(0.1)
Divestitures	(0.1)	(0.5)
Transfer of operations from East Group to West Group	1.9
Balance at end of period	$ 1,841.5	$ 1,822.5