Revenue (Schedule of Net Sales by Major Customer Group within Each Segment) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 975.3	$ 893.6	$ 837.3
Mineral Fiber [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	801.6	756.4	736.6
Mineral Fiber [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	601.4	563.3	549.0
Mineral Fiber [Member] | Home Centers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	84.0	82.2	78.7
Mineral Fiber [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	60.3	62.1	61.9
Mineral Fiber [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Total net sales	55.9	48.8	47.0
Architectural Specialties [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	173.7	137.2	100.7
Architectural Specialties [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	129.8	111.5	73.4
Architectural Specialties [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	36.7	22.3	23.9
Architectural Specialties [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 7.2	$ 3.4	$ 3.4