Revenue (Schedule of Net Sales by Major Customer Group within Each Segment) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 248.7	$ 242.1
Mineral Fiber [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	197.7	196.7
Mineral Fiber [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	141.9	144.6
Mineral Fiber [Member] | Home Centers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	25.7	23.1
Mineral Fiber [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	13.9	14.1
Mineral Fiber [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Total net sales	16.2	14.9
Architectural Specialties [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	51.0	45.4
Architectural Specialties [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	35.9	31.5
Architectural Specialties [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	14.4	13.1
Architectural Specialties [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 0.7	$ 0.8