Nature of Operations (Schedule of Net Sales to External Customers) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Net sales		$ 893.6	$ 837.3	$ 805.1
Equity (earnings) from joint venture		(67.0)	(73.1)	(66.1)
Segment operating income (loss)		255.1	188.9	157.0
Segment assets		1,567.4	1,411.7	1,421.0
Depreciation and amortization	[1]	67.0	54.8	56.6
Investment in joint venture		107.3	106.2	130.8
Purchases of property, plant and equipment	[1]	77.7	66.3	74.5
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Net sales		756.4	736.6	723.7
Equity (earnings) from joint venture		(67.0)	(73.1)	(66.1)
Segment operating income (loss)		231.9	223.9	270.3
Segment assets		1,193.5	1,145.1	1,132.8
Depreciation and amortization	[1]	59.2	53.6	43.9
Investment in joint venture		107.3	106.2	130.8
Purchases of property, plant and equipment	[1]	76.1	66.1	51.5
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Net sales		137.2	100.7	81.4
Segment operating income (loss)		27.7	19.2	16.5
Segment assets		53.2	17.3	17.2
Depreciation and amortization	[1]	1.8	0.8	0.8
Purchases of property, plant and equipment	[1]	1.6	0.2	0.6
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Segment operating income (loss)		(4.5)	(54.2)	(129.8)
Segment assets		320.7	249.3	271.0
Depreciation and amortization	[1]	$ 6.0	$ 0.4	11.9
Purchases of property, plant and equipment	[1]			$ 22.4

[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 4 for additional details.