Nature of Operations (Schedule of Net Sales to External Customers) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Net sales		$ 975.3	$ 893.6	$ 837.3
Equity (earnings) from joint venture		(74.9)	(67.0)	(73.1)
Segment operating income (loss)		249.4	243.8	195.9
Segment assets		1,594.0	1,567.4	1,411.7
Depreciation and amortization	[1]	79.4	67.0	54.8
Investment in joint venture		40.8	107.3	106.2
Purchases of property, plant and equipment	[1]	64.6	77.7	66.3
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Net sales		801.6	756.4	736.6
Equity (earnings) from joint venture		(74.9)	(67.0)	(73.1)
Segment operating income (loss)		223.8	233.5	226.5
Segment assets		1,096.1	1,193.5	1,145.1
Depreciation and amortization	[1]	75.3	59.2	53.6
Investment in joint venture		40.8	107.3	106.2
Purchases of property, plant and equipment	[1]	60.5	76.1	66.1
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Net sales		173.7	137.2	100.7
Segment operating income (loss)		34.3	27.7	19.2
Segment assets		84.7	53.2	17.3
Depreciation and amortization	[1]	3.5	1.8	0.8
Purchases of property, plant and equipment	[1]	4.1	1.6	0.2
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Segment operating income (loss)		(8.7)	(17.4)	(49.8)
Segment assets		413.2	320.7	249.3
Depreciation and amortization	[1]	$ 0.6	$ 6.0	$ 0.4

[1] Totals will differ from the totals on our Consolidated Statement of Cash Flows by the amounts that have been classified as discontinued operations. See Note 5 for additional details.