Segment Information (Measurements Used by Management to Assess Performance of Segments) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net sales:	$ 258,131	$ 299,236	$ 262,655	$ 227,417	$ 240,913	$ 280,086	$ 266,036	$ 215,337	$ 1,047,439	$ 1,002,372	$ 986,918
Income from operations:	19,247	$ 31,420	$ 26,606	$ 10,566	18,447	$ 29,404	$ 32,274	$ 13,843	87,839	93,968	92,849
Depreciation and amortization									19,949	20,374	21,690
Total assets	984,450				1,061,645				984,450	1,061,645	991,385
Capital expenditures									11,184	12,457	11,399
Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Income from operations:									124,060	122,597	115,270
Intersegment sales
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net sales:									(1,019)	(1,103)	(1,247)
Unallocated Corporate Expenses
Segment Reporting, Other Significant Reconciling Item [Line Items]
Income from operations:									(36,221)	(28,629)	(22,421)
Depreciation and amortization									390	332	321
Total assets	174,475				271,616				174,475	271,616	209,286
Capital expenditures									581	175	421
Renewable Energy & Conservation
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net sales:									373,023	317,253	306,351
Renewable Energy & Conservation | Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net sales:									373,023	317,253	306,351
Income from operations:									47,558	37,423	30,218
Depreciation and amortization									6,132	5,790	5,657
Total assets	246,853				218,048				246,853	218,048	219,806
Capital expenditures									2,199	1,345	3,648
Residential Products
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net sales:									461,630	463,216	466,603
Residential Products | Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net sales:									461,630	463,216	466,603
Income from operations:									63,047	69,838	76,893
Depreciation and amortization									7,906	8,217	9,183
Total assets	359,657				361,499				359,657	361,499	358,838
Capital expenditures									4,968	7,921	5,236
Industrial and Infrastructure Products
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net sales:									212,786	221,903	213,964
Industrial and Infrastructure Products | Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net sales:									213,805	223,006	215,211
Income from operations:									13,455	15,336	8,159
Depreciation and amortization									5,521	6,035	6,529
Total assets	$ 203,465				$ 210,482				203,465	210,482	203,455
Capital expenditures									$ 3,436	$ 3,016	$ 2,094