Segment Results (Reconciliation of Total Segment Assets to Total Consolidated Assets) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Total consolidated assets	$ 1,704.9	$ 1,718.5
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	1,119.9	1,101.1
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	365.4	357.7
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	$ 219.6	$ 259.7