Segment Results (Schedule of Segment Operating Income (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Total consolidated operating income	$ 87.2	$ 66.0	$ 141.9	$ 115.6
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	79.4	59.5	127.0	103.2
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	9.5	8.6	18.7	16.9
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Total consolidated operating income	$ (1.7)	$ (2.1)	$ (3.8)	$ (4.5)