Revenue (Schedule of Net Sales by Major Customer Group within Each Segment) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 251.9	$ 248.7
Mineral Fiber [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	188.7	197.7
Mineral Fiber [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	135.3	141.9
Mineral Fiber [Member] | Home Centers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	26.8	25.7
Mineral Fiber [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	12.8	13.9
Mineral Fiber [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Total net sales	13.8	16.2
Architectural Specialties [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	63.2	51.0
Architectural Specialties [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	34.3	35.9
Architectural Specialties [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	28.1	14.4
Architectural Specialties [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 0.8	$ 0.7