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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

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