Segment Results (Schedule of Net Sales) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Total net sales	$ 203.2	$ 272.0	$ 451.9	$ 514.1
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total net sales	157.9	214.1	355.6	410.8
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total net sales	$ 45.3	$ 57.9	$ 96.3	$ 103.3