Segment Results (Reconciliation of Total Segment Assets to Total Consolidated Assets) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total consolidated assets	$ 1,712.2	$ 1,567.4
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	500.8	320.7
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	1,126.7	1,193.5
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total consolidated assets	$ 84.7	$ 53.2