Nature of Operations (Schedule Of Property, Plant And Equipment Allocated To Geographic Area) (Details) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant and equipment, net		$ 501.0	$ 499.9
Mineral Fiber [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant and equipment, net		487.5	488.7
Mineral Fiber [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant and equipment, net		487.5	488.7
Architectural Specialties [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant and equipment, net		10.4	7.5
Architectural Specialties [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant and equipment, net		5.9	3.0
Architectural Specialties [Member] | Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant and equipment, net		4.5	4.5
Unallocated Corporate [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant and equipment, net	[1]	$ 3.1	$ 3.7

[1] Includes property, plant and equipment located in China that were formerly reported in our Pacific Rim segment and will not be included in the sale to Knauf.