Segment Results (Schedule of Segment Operating Income (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Total consolidated operating income	$ 81.3	$ 73.9	$ 196.9	$ 200.4
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	(1.8)	(6.9)	(6.3)	(12.9)
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	71.8	73.1	175.0	192.7
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	$ 11.3	$ 7.7	$ 28.2	$ 20.6