Nature of Operations (Schedule of Net Sales to External Customers) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Net sales		$ 1,038.1	$ 975.3	$ 893.6
Equity (earnings) from joint venture		(96.6)	(74.9)	(67.0)
Segment operating income (loss)		317.4	249.4	243.8
Segment assets		1,493.3	1,594.0	1,567.4
Depreciation and amortization		72.1	79.4	67.0	[1]
Investment in joint venture		58.5	40.8	107.3
Purchases of property, plant and equipment	[1]	68.3	64.6	77.7
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Net sales		826.6	801.6	756.4
Equity (earnings) from joint venture		(96.6)	(74.9)	(67.0)
Segment operating income (loss)		289.6	223.8	233.5
Segment assets		1,139.9	1,096.1	1,193.5
Depreciation and amortization		62.9	75.3	59.2	[1]
Investment in joint venture		58.5	40.8	107.3
Purchases of property, plant and equipment	[1]	59.5	60.5	76.1
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Net sales		211.5	173.7	137.2
Segment operating income (loss)		35.9	34.3	27.7
Segment assets		161.8	84.7	53.2
Depreciation and amortization		8.8	3.5	1.8	[1]
Purchases of property, plant and equipment	[1]	8.8	4.1	1.6
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Segment operating income (loss)		(8.1)	(8.7)	(17.4)
Segment assets		191.6	413.2	320.7
Depreciation and amortization		$ 0.4	$ 0.6	$ 6.0	[1]

[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 7 for additional details.