Revenue (Schedule of Net Sales by Major Customer Group within Each Segment) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Entity Wide Revenue Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 321.0	$ 280.0	$ 603.6	$ 531.9
Mineral Fiber [Member]
Entity Wide Revenue Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	234.5	208.1	437.7	396.8
Mineral Fiber [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	177.7	156.7	321.2	292.0
Mineral Fiber [Member] | Home Centers [Member]
Entity Wide Revenue Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	23.5	21.4	51.6	48.2
Mineral Fiber [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	16.3	15.2	30.6	28.0
Mineral Fiber [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	17.0	14.8	34.3	28.6
Architectural Specialties [Member]
Entity Wide Revenue Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	86.5	71.9	165.9	135.1
Architectural Specialties [Member] | Distributors [Member]
Entity Wide Revenue Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	45.9	40.3	84.7	74.6
Architectural Specialties [Member] | Direct Customers [Member]
Entity Wide Revenue Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	40.3	30.8	79.7	58.9
Architectural Specialties [Member] | Retailers And Other
Entity Wide Revenue Major Customer [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 0.3	$ 0.8	$ 1.5	$ 1.6