Segment Results (Reconciliation of Total Segment Assets to Total Consolidated Assets) (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Total consolidated assets	$ 1,804.0	$ 1,672.4
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	1,087.2	1,091.9
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	544.9	421.1
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	$ 171.9	$ 159.4