Segment Results (Schedule of Net Sales) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Total net sales	$ 292.2	$ 246.3	$ 824.1	$ 698.2
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total net sales	214.5	187.3	611.3	542.9
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total net sales	$ 77.7	$ 59.0	$ 212.8	$ 155.3