Segment Results (Schedule of Segment Operating Income (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Total consolidated operating income	$ 62.4	$ 87.2	$ 138.4	$ 141.9
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	45.6	79.4	115.6	127.0
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	4.3	9.5	11.8	18.7
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	$ 12.5	$ (1.7)	$ 11.0	$ (3.8)