Segment Results (Reconciliation of Total Segment Assets to Total Consolidated Assets) (Details) - USD ($) $ in Millions	Jun. 30, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Total consolidated assets	$ 1,734.0	$ 1,710.0
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	1,153.5	1,133.9
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	385.3	366.3
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	$ 195.2	$ 209.8