Revenue (Schedule of Net Sales by Major Customer Group within Each Segment) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 260.5	$ 233.9	$ 736.4	$ 679.3
Mineral Fiber [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	212.8	199.0	610.2	578.9
Mineral Fiber [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	164.8	152.2	459.2	430.0
Mineral Fiber [Member] | Home Centers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	18.1	18.7	62.6	63.1
Mineral Fiber [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	15.5	15.9	46.0	47.6
Mineral Fiber [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Total net sales	14.4	12.2	42.4	38.2
Architectural Specialties [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	47.7	34.9	126.2	100.4
Architectural Specialties [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	34.3	28.5	96.1	79.9
Architectural Specialties [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	10.8	5.8	26.2	17.9
Architectural Specialties [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 2.6	$ 0.6	$ 3.9	$ 2.6