Segment Results (Reconciliation of Total Segment Assets to Total Consolidated Assets) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Total consolidated assets	$ 1,633.7	$ 1,493.3
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	1,110.9	1,139.9
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	258.8	161.8
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	$ 264.0	$ 191.6