Revenue (Schedule of Net Sales by Major Customer Group within Each Segment) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 246.3	$ 277.1	$ 698.2	$ 791.2
Mineral Fiber [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	187.3	218.6	542.9	629.4
Mineral Fiber [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	138.6	168.0	395.1	474.0
Mineral Fiber [Member] | Home Centers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	22.5	20.0	67.6	64.3
Mineral Fiber [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	13.8	15.8	41.9	47.4
Mineral Fiber [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Total net sales	12.4	14.8	38.3	43.7
Architectural Specialties [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	59.0	58.5	155.3	161.8
Architectural Specialties [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	36.0	39.7	103.9	108.5
Architectural Specialties [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 23.0	16.7	49.8	48.8
Architectural Specialties [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Total net sales		$ 2.1	$ 1.6	$ 4.5