Segment Results (Schedule of Segment Operating Income (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Total consolidated operating income	$ 54.1	$ 76.0
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	60.6	70.0
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	(4.9)	7.5
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	$ (1.6)	$ (1.5)