Acquisitions (Narrative) (Details) - USD ($) $ in Millions	Mar. 04, 2019	Aug. 16, 2018	May 31, 2018	Jun. 30, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Business acquisition, goodwill				$ 40.8	$ 19.2
Architectural Components Group, Inc. [Member]
Business Acquisition [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]
Business Acquisition [Line Items]
Business acquisition, total fair value of intangible assets acquired	7.4
Architectural Components Group, Inc. [Member] | TradeNames [Member]
Business Acquisition [Line Items]
Business acquisition, total fair value of intangible assets acquired	$ 2.8
Steel Ceilings, Inc. [Member]
Business Acquisition [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
Proceed from sale of acquired product		2.0
Steel Ceilings, Inc. [Member] | Customer Relationships [Member]
Business Acquisition [Line Items]
Business acquisition, total fair value of intangible assets acquired		1.4
Steel Ceilings, Inc. [Member] | TradeNames [Member]
Business Acquisition [Line Items]
Business acquisition, total fair value of intangible assets acquired		$ 1.3
Plasterform, Inc. [Member]
Business Acquisition [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