Segment Results (Reconciliation of Total Segment Assets to Total Consolidated Assets) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Total consolidated assets	$ 1,509.7	$ 1,594.0
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	1,141.6	1,096.1
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	149.6	84.7
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	$ 218.5	$ 413.2