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Segment Information - Summary of depreciation and amortization by segment (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Segment Reporting Information [Line Items] |  |  |
Depreciation and amortization | $ 39,913 | $ 38,576 | $ 39,122
T&D [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Depreciation and amortization | 33,977 | 34,990 | 35,947
C&I [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Depreciation and amortization | $ 5,936 | $ 3,586 | $ 3,175

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- Definition The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 22 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585 + Details
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