Acquisition and Discontinued Operations (Narrative) (Details) - USD ($) $ in Millions				3 Months Ended		12 Months Ended
	Mar. 04, 2019	Aug. 16, 2018	May 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Oct. 31, 2018
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Business acquisition, goodwill				$ 40.6		$ 19.2
(Loss) gain due to change in carrying value of discontinued operations net assets				2.2	$ (17.3)
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				2.2	$ (17.7)	(19.3)	$ (74.0)
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				$ 9.1		$ (25.5)	$ (51.4)
Architectural Components Group, Inc. [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Business acquisition, purchase price	$ 43.1
Business acquisition, total fair value of tangible assets acquired, less liabilities assumed	9.9
Business acquisition, total fair value of intangible assets acquired	12.0
Business acquisition, goodwill	21.2
Architectural Components Group, 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	7.4
Architectural Components Group, 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	$ 2.8
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
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			$ 11.9
Business acquisition, total fair value of tangible assets acquired, less liabilities assumed			2.2
Business acquisition, total fair value of intangible assets acquired			4.8
Business acquisition, goodwill			$ 4.9