Segment Results (Schedule of Segment Operating Income (Loss)) (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 Consolidated Operating Income (Loss)	$ 72.1	$ 72.3	$ 204.5	$ 210.7
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total Consolidated Operating Income (Loss)	68.5	58.1	201.2	173.7
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total Consolidated Operating Income (Loss)	5.0	9.1	7.5	20.9
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Total Consolidated Operating Income (Loss)	$ (1.4)	$ 5.1	$ (4.2)	$ 16.1