Revenue (Schedule of Net Sales by Major Customer Group within Each Segment) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 1,038.1	$ 975.3	$ 893.6
Mineral Fiber [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	826.6	801.6	756.4
Mineral Fiber [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	622.9	601.4	563.3
Mineral Fiber [Member] | Home Centers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	85.2	84.0	82.2
Mineral Fiber [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	61.2	60.3	62.1
Mineral Fiber [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Total net sales	57.3	55.9	48.8
Architectural Specialties [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	211.5	173.7	137.2
Architectural Specialties [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	138.3	129.8	111.5
Architectural Specialties [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	68.0	36.7	22.3
Architectural Specialties [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 5.2	$ 7.2	$ 3.4