Nature of Operations (Schedule Of Property, Plant And Equipment Allocated To Geographic Area) (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant and equipment, net		$ 499.9	$ 465.2
Mineral Fiber [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant and equipment, net		488.7	457.3
Mineral Fiber [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant and equipment, net		488.7	457.3
Architectural Specialties [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant and equipment, net		7.5	4.3
Architectural Specialties [Member] | United States [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant and equipment, net		3.0	0.3
Architectural Specialties [Member] | Canada [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant and equipment, net		4.5	4.0
Unallocated Corporate [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Property, plant and equipment, net	[1]	$ 3.7	$ 3.6

[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.