Revenue (Schedule of Net Sales by Major Customer Group within Each Segment) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 203.2	$ 272.0	$ 451.9	$ 514.1
Mineral Fiber [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	157.9	214.1	355.6	410.8
Mineral Fiber [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	114.6	161.4	256.5	306.0
Mineral Fiber [Member] | Home Centers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	19.4	21.2	45.1	44.3
Mineral Fiber [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	14.2	17.5	28.1	31.6
Mineral Fiber [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Total net sales	9.7	14.0	25.9	28.9
Architectural Specialties [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	45.3	57.9	96.3	103.3
Architectural Specialties [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	32.0	37.3	67.9	68.8
Architectural Specialties [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	12.4	19.0	26.8	32.1
Architectural Specialties [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 0.9	$ 1.6	$ 1.6	$ 2.4