Segment Results (Schedule of Segment Operating Income (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Segment Reporting Information [Line Items]
Total Consolidated Operating Income (Loss)	$ 71.6	$ 78.3	$ 134.8	$ 132.4
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total Consolidated Operating Income (Loss)	71.4	72.1	129.1	132.7
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total Consolidated Operating Income (Loss)	1.1	7.4	7.6	2.5
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Total Consolidated Operating Income (Loss)	$ (0.9)	$ (1.2)	$ (1.9)	$ (2.8)