Segment Results (Schedule of Segment Operating Income (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Total consolidated operating income	$ 113.3	$ 81.3	$ 255.2	$ 196.9
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	103.5	71.8	230.5	175.0
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	11.6	11.3	30.3	28.2
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	$ (1.8)	$ (1.8)	$ (5.6)	$ (6.3)