Segment Results (Schedule of Net Sales) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Total net sales	$ 246.3	$ 277.1	$ 698.2	$ 791.2
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total net sales	187.3	218.6	542.9	629.4
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total net sales	$ 59.0	$ 58.5	$ 155.3	$ 161.8