Segment Results (Schedule of Net Sales) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Total net sales	$ 251.9	$ 248.7
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total net sales	188.7	197.7
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total net sales	$ 63.2	$ 51.0