Segment Results (Reconciliation of Total Consolidated Operating Income to Earnings Before Income Taxes) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Segment Reporting Information [Line Items]
Total Consolidated Operating Income (Loss)	$ 95.0	$ 87.0	$ 181.1	$ 157.2
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total Consolidated Operating Income (Loss)	81.7	75.5	160.9	139.3
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total Consolidated Operating Income (Loss)	14.2	12.2	21.9	19.4
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Total Consolidated Operating Income (Loss)	$ (0.9)	$ (0.7)	$ (1.7)	$ (1.5)