Segment Results (Schedule of Segment Operating Income (Loss)) (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 consolidated operating income	$ 72.3	$ 113.3	$ 210.7	$ 255.2
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	58.1	103.5	173.7	230.5
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	9.1	11.6	20.9	30.3
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	$ 5.1	$ (1.8)	$ 16.1	$ (5.6)