Segment Results (Schedule of Net Sales) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Total net sales	$ 280.0	$ 203.2	$ 531.9	$ 451.9
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total net sales	208.1	157.9	396.8	355.6
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total net sales	$ 71.9	$ 45.3	$ 135.1	$ 96.3