Segment Information (Net Sales by Contract Type) (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
Disaggregation of Revenue [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
Renewable Energy & Conservation
Disaggregation of Revenue [Line Items]
Net sales:									373,023	317,253	306,351
Residential Products
Disaggregation of Revenue [Line Items]
Net sales:									461,630	463,216	466,603
Industrial and Infrastructure Products
Disaggregation of Revenue [Line Items]
Net sales:									212,786	221,903	213,964
Point in Time
Disaggregation of Revenue [Line Items]
Net sales:									676,298	682,021	710,704
Point in Time | Renewable Energy & Conservation
Disaggregation of Revenue [Line Items]
Net sales:									42,596	33,427	30,137
Point in Time | Residential Products
Disaggregation of Revenue [Line Items]
Net sales:									458,006	460,513	466,603
Point in Time | Industrial and Infrastructure Products
Disaggregation of Revenue [Line Items]
Net sales:									175,696	188,081	213,964
Over Time
Disaggregation of Revenue [Line Items]
Net sales:									371,141	320,351	276,214
Over Time | Renewable Energy & Conservation
Disaggregation of Revenue [Line Items]
Net sales:									330,427	283,826	276,214
Over Time | Residential Products
Disaggregation of Revenue [Line Items]
Net sales:									3,624	2,703	0
Over Time | Industrial and Infrastructure Products
Disaggregation of Revenue [Line Items]
Net sales:									$ 37,090	$ 33,822	$ 0