Segment Results (Reconciliation of Total Segment Assets to Total Consolidated Assets) (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Total consolidated assets	$ 1,708.1	$ 1,710.0
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	1,142.7	1,133.9
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	378.1	366.3
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	$ 187.3	$ 209.8