Nature of Operations (Schedule Of Sales Allocated To Geographic Area) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues From External Customers And Long Lived Assets [Line Items]
Net trade sales	$ 893.6	$ 837.3	$ 805.1
Mineral Fiber [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net trade sales	756.4	736.6	723.7
Mineral Fiber [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net trade sales	699.8	680.8	670.8
Mineral Fiber [Member] | Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net trade sales	56.6	55.8	52.9
Architectural Specialties [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net trade sales	137.2	100.7	81.4
Architectural Specialties [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net trade sales	129.5	95.1	75.1
Architectural Specialties [Member] | Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net trade sales	$ 7.7	$ 5.6	$ 6.3