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Accounts receivable and allowance for doubtful accounts (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
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Accounting Policies [Abstract] |  |
Receivables past due 90 days or more | $ 30,000 | $ 34,700
Allowance for doubtful accounts | 8,900 | 8,100
Accrued unbilled revenue at Montana-Dakota, Cascade and Intermountain | 96,200 | 112,700
Short-term retainage | 56,228 | 57,134
Long-term retainage | 4,152 | 1,410
Total retainage | $ 60,380 | $ 58,544

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