Segment Results (Reconciliation of Total Segment Assets to Total Consolidated Assets) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Total consolidated assets	$ 1,590.3	$ 1,493.3
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	1,141.9	1,139.9
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	161.3	161.8
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	$ 287.1	$ 191.6