Nature of Operations (Schedule Of Sales Allocated To Geographic Area) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues From External Customers And Long Lived Assets [Line Items]
Net trade sales	$ 975.3	$ 893.6	$ 837.3
Mineral Fiber [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net trade sales	801.6	756.4	736.6
Mineral Fiber [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net trade sales	739.2	699.8	680.8
Mineral Fiber [Member] | Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net trade sales	62.4	56.6	55.8
Architectural Specialties [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net trade sales	173.7	137.2	100.7
Architectural Specialties [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net trade sales	157.5	129.5	95.1
Architectural Specialties [Member] | Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Net trade sales	$ 16.2	$ 7.7	$ 5.6