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Segment Information (Net Sales by Contract Type) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Net sales:	$ 240,913	$ 280,086	$ 266,036	$ 215,337	$ 258,112	$ 274,574	$ 247,627	$ 206,605	$ 1,002,372	$ 986,918	$ 1,007,981
Residential Products
Disaggregation of Revenue [Line Items]
Net sales:									463,216	466,603	430,938
Industrial and Infrastructure Products
Disaggregation of Revenue [Line Items]
Net sales:									221,903	213,964	295,018
Renewable Energy & Conservation
Disaggregation of Revenue [Line Items]
Net sales:									317,253	306,351	282,025
Point in Time
Disaggregation of Revenue [Line Items]
Net sales:									682,021	710,704	747,522
Point in Time | Residential Products
Disaggregation of Revenue [Line Items]
Net sales:									460,513	466,603	430,938
Point in Time | Industrial and Infrastructure Products
Disaggregation of Revenue [Line Items]
Net sales:									188,081	213,964	295,018
Point in Time | Renewable Energy & Conservation
Disaggregation of Revenue [Line Items]
Net sales:									33,427	30,137	21,566
Over Time
Disaggregation of Revenue [Line Items]
Net sales:									320,351	276,214	260,459
Over Time | Residential Products
Disaggregation of Revenue [Line Items]
Net sales:									2,703	0	0
Over Time | Industrial and Infrastructure Products
Disaggregation of Revenue [Line Items]
Net sales:									33,822	0	0
Over Time | Renewable Energy & Conservation
Disaggregation of Revenue [Line Items]
Net sales:									$ 283,826	$ 276,214	$ 260,459

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (a)
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