Revenue (Schedule of Net Sales by Major Customer Group within Each Segment) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 242.1	$ 227.3
Mineral Fiber [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	196.7	190.7
Mineral Fiber [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	144.6	137.7
Mineral Fiber [Member] | Home Centers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	23.1	24.0
Mineral Fiber [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	14.1	13.7
Mineral Fiber [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Total net sales	14.9	15.3
Architectural Specialties [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	45.4	36.6
Architectural Specialties [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	31.5	28.4
Architectural Specialties [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	13.1	7.9
Architectural Specialties [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 0.8	$ 0.3