Segment Results (Schedule of Net Sales by Major Customer Group within Each Segment) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 227.3	$ 219.8
Mineral Fiber [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	190.7	189.8
Mineral Fiber [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	137.7	140.2
Mineral Fiber [Member] | U.S Home Centers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	24.0	25.9
Mineral Fiber [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	13.7	16.1
Mineral Fiber [Member] | Retailers / Other [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	15.3	7.6
Architectural Specialties [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	36.6	30.0
Architectural Specialties [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	28.4	23.4
Architectural Specialties [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	7.9	5.6
Architectural Specialties [Member] | Retailers / Other [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 0.3	$ 1.0