Segment Results (Reconciliation of Total Segment Assets to Total Consolidated Assets) (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Segment Reporting Information [Line Items]
Segment assets	$ 1,854.4	$ 1,842.7
Investments in unconsolidated affiliates	28.9	27.2
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Segment assets	1,080.2	1,063.8
Investments in unconsolidated affiliates	24.6	22.6
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Segment assets	596.7	602.2
Investments in unconsolidated affiliates	0.0	0.0
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Segment assets	177.5	176.7
Investments in unconsolidated affiliates	$ 4.3	$ 4.6