Segment Results (Schedule of Segment Operating Income (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Total consolidated operating income	$ 49.6	$ 57.3
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	(2.4)	(3.0)
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	43.7	55.5
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	$ 8.3	$ 4.8