Revenue (Schedule of Net Sales by Major Customer Group within Each Segment) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 272.0	$ 248.6	$ 514.1	$ 475.9
Mineral Fiber [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	214.1	206.7	410.8	397.4
Mineral Fiber [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	161.4	156.7	306.0	294.4
Mineral Fiber [Member] | Home Centers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	21.2	20.5	44.3	44.5
Mineral Fiber [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	17.5	16.8	31.6	30.5
Mineral Fiber [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Total net sales	14.0	12.7	28.9	28.0
Architectural Specialties [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	57.9	41.9	103.3	78.5
Architectural Specialties [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	37.3	33.4	68.8	61.8
Architectural Specialties [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	19.0	7.5	32.1	15.4
Architectural Specialties [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 1.6	$ 1.0	$ 2.4	$ 1.3