Segment Information (Net Sales by Contract Type) (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
Disaggregation of Revenue [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
Renewable Energy & Conservation
Disaggregation of Revenue [Line Items]
Net sales:									447,567	373,023	317,253
Residential Products
Disaggregation of Revenue [Line Items]
Net sales:									522,814	461,630	463,216
Infrastructure Products
Disaggregation of Revenue [Line Items]
Net sales:									62,197	63,580	56,628
Point in Time
Disaggregation of Revenue [Line Items]
Net sales:									616,789	527,092	516,746
Point in Time | Renewable Energy & Conservation
Disaggregation of Revenue [Line Items]
Net sales:									75,727	42,596	33,427
Point in Time | Residential Products
Disaggregation of Revenue [Line Items]
Net sales:									518,281	458,006	460,513
Point in Time | Infrastructure Products
Disaggregation of Revenue [Line Items]
Net sales:									22,781	26,490	22,806
Over Time
Disaggregation of Revenue [Line Items]
Net sales:									415,789	371,141	320,351
Over Time | Renewable Energy & Conservation
Disaggregation of Revenue [Line Items]
Net sales:									371,840	330,427	283,826
Over Time | Residential Products
Disaggregation of Revenue [Line Items]
Net sales:									4,533	3,624	2,703
Over Time | Infrastructure Products
Disaggregation of Revenue [Line Items]
Net sales:									$ 39,416	$ 37,090	$ 33,822