Revenue (Schedule of Net Sales by Major Customer Group within Each Segment) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 280.0	$ 203.2	$ 531.9	$ 451.9
Mineral Fiber [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	208.1	157.9	396.8	355.6
Mineral Fiber [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	156.7	114.6	292.0	256.5
Mineral Fiber [Member] | Home Centers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	21.4	19.4	48.2	45.1
Mineral Fiber [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	15.2	14.2	28.0	28.1
Mineral Fiber [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Total net sales	14.8	9.7	28.6	25.9
Architectural Specialties [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	71.9	45.3	135.1	96.3
Architectural Specialties [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	40.3	32.0	74.6	67.9
Architectural Specialties [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	30.8	12.4	58.9	26.8
Architectural Specialties [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 0.8	$ 0.9	$ 1.6	$ 1.6