Acquisition and Discontinued Operations (Narrative) (Details) - USD ($) $ in Millions		1 Months Ended			3 Months Ended			6 Months Ended	9 Months Ended
	Aug. 16, 2018	May 31, 2018	Apr. 30, 2017	Jan. 31, 2017	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Oct. 31, 2018
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Business acquisition, goodwill					$ 19.4	$ 11.3			$ 19.4
(Loss) gain due to change in carrying value of discontinued operations net assets					7.0		$ 5.9		(16.1)	$ 5.3
Income tax benefit related to discontinued operations					4.6		$ 5.9		4.9	$ 5.4
EMEA and Pacific Rim Business [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
(Loss) gain due to change in carrying value of discontinued operations net assets					2.4	(74.0)		$ (23.4)	(21.0)
EMEA and Pacific Rim Business [Member] | Foreign Currency Translation Adjustments [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
(Loss) gain due to change in carrying value of discontinued operations net assets					$ 7.6	$ 51.4		$ 13.9	$ 21.5
DLW [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Settlement agreement and mutual release with administrator to settle all claims of Administrator related to insolvency for a cash payment			$ 11.8
Steel Ceilings, Inc. [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Business acquisition, purchase price	$ 12.3
Business acquisition, total fair value of tangible assets acquired, less liabilities assumed	4.4
Business acquisition, goodwill	3.2
Steel Ceilings, Inc. [Member] | Subsequent Event [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Assets Held-for-sale, Not Part of Disposal Group, Current, Other											$ 2.0
Steel Ceilings, Inc. [Member] | Customer Relationships [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Business acquisition, total fair value of intangible assets acquired	1.4
Steel Ceilings, Inc. [Member] | TradeNames [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Business acquisition, total fair value of intangible assets acquired	$ 1.3
Plasterform, Inc. [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Business acquisition, purchase price		$ 11.6
Business acquisition, total fair value of tangible assets acquired, less liabilities assumed		1.9
Business acquisition, total fair value of intangible assets acquired		4.8
Business acquisition, goodwill		$ 4.9
Tectum [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Business acquisition, purchase price				$ 31.2
Business acquisition, total fair value of tangible assets acquired, less liabilities assumed				4.4
Business acquisition, total fair value of intangible assets acquired				16.0
Business acquisition, goodwill				$ 10.8