Goodwill and Other Intangible Assets - Goodwill and Accumulated Goodwill Impairment Losses (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Goodwill [Roll Forward]
Goodwill, gross, beginning balance	$ 2,640.5	$ 2,640.5	$ 2,510.2
Accumulated goodwill impairment losses, beginning balance	(1,369.8)	(1,369.8)	(1,281.8)
Goodwill, net, beginning balance	1,270.7	1,270.7	1,228.4
Goodwill acquired during year			111.6
Impairment losses during year	(113.0)	(113.0)	(88.0)
Other		(0.6)	18.7
Goodwill, gross, ending balance		2,639.9	2,640.5
Accumulated goodwill impairment losses, ending balance		(1,482.8)	(1,369.8)
Goodwill, net, ending balance		1,157.1	1,270.7
Northeast segment
Goodwill [Roll Forward]
Impairment losses during year		(10.3)
South segment
Goodwill [Roll Forward]
Impairment losses during year		(17.4)
Midwest segment
Goodwill [Roll Forward]
Impairment losses during year		(60.3)
Operating segments | Northeast segment
Goodwill [Roll Forward]
Goodwill, gross, beginning balance	914.3	914.3	848.4
Accumulated goodwill impairment losses, beginning balance	(717.9)	(717.9)	(707.6)
Goodwill, net, beginning balance	196.4	196.4	140.8
Goodwill acquired during year			67.4
Impairment losses during year	(43.5)	(43.5)	(10.3)
Other		0.0	(1.5)
Goodwill, gross, ending balance		914.3	914.3
Accumulated goodwill impairment losses, ending balance		(761.4)	(717.9)
Goodwill, net, ending balance		152.9	196.4
Operating segments | South segment
Goodwill [Roll Forward]
Goodwill, gross, beginning balance	236.6	236.6	185.2
Accumulated goodwill impairment losses, beginning balance	(52.0)	(52.0)	(34.6)
Goodwill, net, beginning balance	184.6	184.6	150.6
Goodwill acquired during year			44.2
Impairment losses during year	(9.0)	(9.0)	(17.4)
Other		0.0	7.2
Goodwill, gross, ending balance		236.6	236.6
Accumulated goodwill impairment losses, ending balance		(61.0)	(52.0)
Goodwill, net, ending balance		175.6	184.6
Operating segments | West segment
Goodwill [Roll Forward]
Goodwill, gross, beginning balance	216.8	216.8	210.4
Accumulated goodwill impairment losses, beginning balance	(16.6)	(16.6)	(16.6)
Goodwill, net, beginning balance	200.2	200.2	193.8
Goodwill acquired during year			0.0
Impairment losses during year		0.0	0.0
Other		0.0	6.4
Goodwill, gross, ending balance		216.8	216.8
Accumulated goodwill impairment losses, ending balance		(16.6)	(16.6)
Goodwill, net, ending balance		200.2	200.2
Operating segments | Midwest segment
Goodwill [Roll Forward]
Goodwill, gross, beginning balance	1,116.7	1,116.7	1,110.1
Accumulated goodwill impairment losses, beginning balance	(495.6)	(495.6)	(435.3)
Goodwill, net, beginning balance	621.1	621.1	674.8
Goodwill acquired during year			0.0
Impairment losses during year	(60.5)	(60.5)	(60.3)
Other		0.0	6.6
Goodwill, gross, ending balance		1,116.7	1,116.7
Accumulated goodwill impairment losses, ending balance		(556.1)	(495.6)
Goodwill, net, ending balance		560.6	621.1
Other
Goodwill [Roll Forward]
Goodwill, gross, beginning balance	156.1	156.1	156.1
Accumulated goodwill impairment losses, beginning balance	(87.7)	(87.7)	(87.7)
Goodwill, net, beginning balance	$ 68.4	68.4	68.4
Goodwill acquired during year			0.0
Impairment losses during year		0.0	0.0
Other		(0.6)	0.0
Goodwill, gross, ending balance		155.5	156.1
Accumulated goodwill impairment losses, ending balance		(87.7)	(87.7)
Goodwill, net, ending balance		$ 67.8	$ 68.4