Segment Results (Schedule of Segment Operating Income (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Segment Reporting Information [Line Items]
Total Consolidated Operating Income (Loss)	$ 87.0	$ 71.6	$ 157.2	$ 134.8
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total Consolidated Operating Income (Loss)	75.5	71.4	139.3	129.1
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total Consolidated Operating Income (Loss)	12.2	1.1	19.4	7.6
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Total Consolidated Operating Income (Loss)	$ (0.7)	$ (0.9)	$ (1.5)	$ (1.9)