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Segment Information (Measurements Used by Management to Assess Performance of Segments) (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	Jan. 01, 2018
Segment Reporting, Other Significant Reconciling Item [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
Income from operations:	18,447	$ 29,404	$ 32,274	$ 13,843	22,547	$ 35,693	$ 24,930	$ 9,679	93,968	92,849	73,488
Depreciation and amortization									20,374	21,690	24,114
Total assets	1,061,645				991,385				1,061,645	991,385	918,245	$ 991,572
Capital expenditures									12,457	11,399	10,779
Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Income from operations:									122,597	115,270	109,761
Intersegment sales
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net sales:									(1,103)	(1,247)	(1,495)
Unallocated Corporate Expenses
Segment Reporting, Other Significant Reconciling Item [Line Items]
Income from operations:									(28,629)	(22,421)	(36,273)
Depreciation and amortization									332	321	377
Total assets	271,616				209,286				271,616	209,286	153,338
Capital expenditures									175	421	377
Residential Products
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net sales:									463,216	466,603	430,938
Residential Products | Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net sales:									463,216	466,603	430,938
Income from operations:									69,838	76,893	65,241
Depreciation and amortization									8,217	9,183	9,297
Total assets	361,499				358,838				361,499	358,838	331,975
Capital expenditures									7,921	5,236	5,182
Industrial and Infrastructure Products
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net sales:									221,903	213,964	295,018
Industrial and Infrastructure Products | Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net sales:									223,006	215,211	296,513
Income from operations:									15,336	8,159	1,306
Depreciation and amortization									6,035	6,529	8,237
Total assets	210,482				203,455				210,482	203,455	225,691
Capital expenditures									3,016	2,094	2,060
Renewable Energy & Conservation
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net sales:									317,253	306,351	282,025
Renewable Energy & Conservation | Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net sales:									317,253	306,351	282,025
Income from operations:									37,423	30,218	43,214
Depreciation and amortization									5,790	5,657	6,203
Total assets	$ 218,048				$ 219,806				218,048	219,806	207,241
Capital expenditures									$ 1,345	$ 3,648	$ 3,160

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