Segment Results (Schedule of Segment Operating Income (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Total consolidated operating income	$ 66.0	$ 69.2	$ 115.6	$ 126.5
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	(2.1)	(3.0)	(4.5)	(6.0)
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	59.5	64.1	103.2	119.6
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	$ 8.6	$ 8.1	$ 16.9	$ 12.9