Segment Results (Reconciliation of Total Segment Assets to Total Consolidated Assets) (Details) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Total consolidated assets	$ 1,739.4	$ 1,710.0
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	1,132.6	1,133.9
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	388.7	366.3
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	$ 218.1	$ 209.8