Segment Results (Schedule of Segment Operating Income (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Total consolidated operating income	$ 78.3	$ 62.4	$ 132.4	$ 138.4
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	72.1	45.6	132.7	115.6
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	7.4	4.3	2.5	11.8
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	$ (1.2)	$ 12.5	$ (2.8)	$ 11.0