Revenue (Schedule of Net Sales by Major Customer Group within Each Segment) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 248.6	$ 225.6	$ 475.9	$ 445.4
Mineral Fiber [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	206.7	190.1	397.4	379.9
Mineral Fiber [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	156.7	141.6	294.4	277.8
Mineral Fiber [Member] | Home Centers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	20.5	18.7	44.5	44.4
Mineral Fiber [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	16.8	16.8	30.5	31.7
Mineral Fiber [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Total net sales	12.7	13.0	28.0	26.0
Architectural Specialties [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	41.9	35.5	78.5	65.5
Architectural Specialties [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	33.4	28.0	61.8	51.4
Architectural Specialties [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Total net sales	7.5	6.5	15.4	12.1
Architectural Specialties [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Total net sales	$ 1.0	$ 1.0	$ 1.3	$ 2.0