Revenue (Schedule of Net Sales by Major Customer Group within Each Segment) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 277.1	$ 260.5	$ 791.2	$ 736.4
Mineral Fiber [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	218.6	212.8	629.4	610.2
Mineral Fiber [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	168.0	164.8	474.0	459.2
Mineral Fiber [Member] | Home Centers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	20.0	18.1	64.3	62.6
Mineral Fiber [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	15.8	15.5	47.4	46.0
Mineral Fiber [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Total net sales	14.8	14.4	43.7	42.4
Architectural Specialties [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	58.5	47.7	161.8	126.2
Architectural Specialties [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	39.7	34.3	108.5	96.1
Architectural Specialties [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	16.7	10.8	48.8	26.2
Architectural Specialties [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 2.1	$ 2.6	$ 4.5	$ 3.9