Nature of Operations (Narrative) (Details) $ in Millions	3 Months Ended	12 Months Ended
Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Costs of goods sold for accelerated depreciation of machinery and equipment		$ 5.6
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Costs of goods sold for accelerated depreciation of machinery and equipment	$ 4.0
WAVE [Member]
Segment Reporting Information [Line Items]
Equity interest percentage	50.00%	50.00%

The expense recognized in the current period that allocates the cost of a tangible asset over the asset's useful life. This element applies only to an asset used in the production of goods sold.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.2(a))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

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- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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