Goodwill and Other Intangible Assets - Changes in Goodwill (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Line Items]
Balance at beginning of period	$ 2,399.1	$ 2,160.3
Measurement period adjustments	(1.6)
Acquisitions		245.3
Goodwill allocated to assets held for sale	(0.2)	(5.6)
Divestitures	(0.5)	(0.9)
Balance at end of period	2,396.8	2,399.1
Mid-America Group
Goodwill [Line Items]
Balance at beginning of period	431.9	281.4
Measurement period adjustments	(1.0)
Acquisitions		150.5
Balance at end of period	430.9	431.9
Southeast Group
Goodwill [Line Items]
Balance at beginning of period	144.2	50.3
Measurement period adjustments	(0.6)
Acquisitions		94.8
Goodwill allocated to assets held for sale	(0.2)
Divestitures		(0.9)
Balance at end of period	143.4	144.2
West Group
Goodwill [Line Items]
Balance at beginning of period	1,823.0	1,828.6
Goodwill allocated to assets held for sale		(5.6)
Divestitures	(0.5)
Balance at end of period	$ 1,822.5	$ 1,823.0