Segment Results (Schedule of Net Sales) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Segment Reporting Information [Line Items]
Total net sales	$ 321.0	$ 280.0	$ 603.6	$ 531.9
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total net sales	234.5	208.1	437.7	396.8
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total net sales	$ 86.5	$ 71.9	$ 165.9	$ 135.1