Nature of Operations (Schedule Of Property, Plant And Equipment Allocated To Geographic Area) (Details) - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant and equipment, net		$ 524.6	$ 501.0
Mineral Fiber [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant and equipment, net		496.6	487.5
Mineral Fiber [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant and equipment, net		496.6	487.5
Architectural Specialties [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant and equipment, net		28.0	10.4
Architectural Specialties [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant and equipment, net		22.2	5.9
Architectural Specialties [Member] | Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant and equipment, net		$ 5.8	4.5
Unallocated Corporate [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant and equipment, net	[1]		$ 3.1

[1] Includes property, plant and equipment located in China that were formerly reported in our Pacific Rim segment that were not included in the Sale. As of December 31, 2019, the property, plant and equipment located in China are classified as an asset held for sale, as we entered into a sale agreement during the third quarter of 2019 with closing expected in the second quarter of 2020.