Segment Results (Reconciliation of Total Segment Assets to Total Consolidated Assets) (Details) - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Segment Reporting Information [Line Items]
Segment assets	$ 2,006.8	$ 1,924.7
Investments in unconsolidated affiliates	29.6	26.6
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Segment assets	1,104.0	1,095.6
Investments in unconsolidated affiliates	26.3	22.9
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Segment assets	712.4	612.7
Investments in unconsolidated affiliates	0.0	0.0
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Segment assets	190.4	216.4
Investments in unconsolidated affiliates	$ 3.3	$ 3.7

Amount of asset recognized for present right to economic benefit.

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+ Details