Segment Information (Measurements Used by Management to Assess Performance of Segments) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net sales:	$ 265,201	$ 296,792	$ 255,184	$ 215,401	$ 226,034	$ 260,784	$ 224,255	$ 187,160	$ 1,032,578	$ 898,233	$ 837,097
Income from operations:	22,722	$ 40,943	$ 30,748	$ 12,777	20,233	$ 28,314	$ 25,073	$ 7,192	107,190	80,812	81,141
Depreciation and amortization									20,915	17,557	17,439
Total assets	1,212,494				984,450				1,212,494	984,450	1,061,645
Capital expenditures									13,068	8,776	10,368
Discontinued Operations
Segment Reporting, Other Significant Reconciling Item [Line Items]
Total assets	77,438				92,854				77,438	92,854	96,128
Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Income from operations:									142,401	117,033	109,770
Unallocated Corporate Expenses
Segment Reporting, Other Significant Reconciling Item [Line Items]
Income from operations:									(35,211)	(36,221)	(28,629)
Depreciation and amortization									300	390	332
Total assets	28,057				174,475				28,057	174,475	271,616
Capital expenditures									7,101	581	175
Renewable Energy & Conservation
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net sales:									447,567	373,023	317,253
Renewable Energy & Conservation | Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net sales:									447,567	373,023	317,253
Income from operations:									40,738	47,558	37,423
Depreciation and amortization									9,445	6,132	5,790
Total assets	619,071				246,853				619,071	246,853	218,048
Capital expenditures									1,143	2,199	1,345
Residential Products
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net sales:									522,814	461,630	463,216
Residential Products | Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net sales:									522,814	461,630	463,216
Income from operations:									94,430	63,047	69,838
Depreciation and amortization									8,110	7,906	8,217
Total assets	407,132				359,657				407,132	359,657	361,499
Capital expenditures									3,313	4,968	7,921
Infrastructure Products
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net sales:									62,197	63,580	56,628
Infrastructure Products | Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Net sales:									62,197	63,580	56,628
Income from operations:									7,233	6,428	2,509
Depreciation and amortization									3,060	3,129	3,100
Total assets	$ 80,796				$ 110,611				80,796	110,611	114,354
Capital expenditures									$ 1,511	$ 1,028	$ 927