Segment Results (Schedule of Segment Operating Income (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Total consolidated operating income	$ 54.7	$ 49.6
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	47.6	43.7
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	9.2	8.3
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	$ (2.1)	$ (2.4)