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Major Customers - Sales revenue for customers (Details) - USD ($) | 3 Months Ended | 12 Months Ended
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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, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Major customers: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total sales | $ 60,654,000 | $ 59,873,000 | $ 37,806,000 | $ 64,023,000 | $ 56,122,000 | $ 74,655,000 | $ 81,247,000 | $ 72,266,000 | $ 73,162,000 | $ 64,676,000 | $ 68,601,000 | $ 63,046,000 | $ 222,356,000 | $ 284,290,000 | $ 269,485,000
Navistar |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Major customers: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total sales |  |  |  |  |  |  |  |  |  |  |  |  | 40,225,000 | 56,882,000 | 55,153,000
Universal Forest Products |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Major customers: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total sales |  |  |  |  |  |  |  |  |  |  |  |  | 21,931,000 | 20,982,000 | 15,798,000
Volvo |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Major customers: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total sales |  |  |  |  |  |  |  |  |  |  |  |  | 28,504,000 | 46,068,000 | 44,452,000
Paccar |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Major customers: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total sales |  |  |  |  |  |  |  |  |  |  |  |  | 25,724,000 | 48,749,000 | 46,160,000
Other customers |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Major customers: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total sales |  |  |  |  |  |  |  |  |  |  |  |  | 67,442,000 | 86,214,000 | 79,776,000
Customer Concentration Risk | Sales Revenue, Total | Universal Forest Products |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Major customers: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total sales |  |  |  |  |  |  |  |  |  |  |  |  | 38,530,000 | 25,395,000 | 28,146,000
Product |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Major customers: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total sales | 58,563,000 | 54,240,000 | 35,847,000 | 61,930,000 | 54,585,000 | 67,511,000 | 75,440,000 | 71,451,000 | 68,975,000 | 62,305,000 | 65,225,000 | 59,712,000 | 210,580,000 | 268,987,000 | 256,217,000
Product | Navistar |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Major customers: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total sales |  |  |  |  |  |  |  |  |  |  |  |  | 33,656,000 | 54,798,000 | 52,347,000
Product | Universal Forest Products |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Major customers: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total sales |  |  |  |  |  |  |  |  |  |  |  |  | 20,269,000 | 16,774,000 | 13,629,000
Product | Volvo |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Major customers: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total sales |  |  |  |  |  |  |  |  |  |  |  |  | 27,997,000 | 44,543,000 | 38,027,000
Product | Paccar |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Major customers: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total sales |  |  |  |  |  |  |  |  |  |  |  |  | 23,538,000 | 48,487,000 | 46,063,000
Product | Other customers |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Major customers: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total sales |  |  |  |  |  |  |  |  |  |  |  |  | 66,590,000 | 78,990,000 | 78,245,000
Product | Customer Concentration Risk | Sales Revenue, Total | Universal Forest Products |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Major customers: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total sales |  |  |  |  |  |  |  |  |  |  |  |  | 38,530,000 | 25,395,000 | 27,906,000
Tooling |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Major customers: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total sales | $ 2,091,000 | $ 5,633,000 | $ 1,959,000 | $ 2,093,000 | $ 1,537,000 | $ 7,144,000 | $ 5,807,000 | $ 815,000 | $ 4,187,000 | $ 2,371,000 | $ 3,376,000 | $ 3,334,000 | 11,776,000 | 15,303,000 | 13,268,000
Tooling | Navistar |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Major customers: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total sales |  |  |  |  |  |  |  |  |  |  |  |  | 6,569,000 | 2,084,000 | 2,806,000
Tooling | Universal Forest Products |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Major customers: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total sales |  |  |  |  |  |  |  |  |  |  |  |  | 1,662,000 | 4,208,000 | 2,169,000
Tooling | Volvo |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Major customers: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total sales |  |  |  |  |  |  |  |  |  |  |  |  | 507,000 | 1,525,000 | 6,425,000
Tooling | Paccar |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Major customers: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total sales |  |  |  |  |  |  |  |  |  |  |  |  | 2,186,000 | 262,000 | 97,000
Tooling | Other customers |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Major customers: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total sales |  |  |  |  |  |  |  |  |  |  |  |  | 852,000 | 7,224,000 | 1,531,000
Tooling | Customer Concentration Risk | Sales Revenue, Total | Universal Forest Products |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Major customers: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total sales |  |  |  |  |  |  |  |  |  |  |  |  | $ 0 | $ 0 | $ 240,000

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