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Accounts receivable and allowance for doubtful accounts (Details) - USD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018
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Accounting Policies [Abstract] |  |
Receivables past due 90 days or more | $ 46,700 | $ 30,000
Allowance for doubtful accounts | 8,500 | 8,900
Accrued unbilled revenue at MDU Energy Capital | 100,800 | 96,200
Short-term retainage | 75,590 | 56,228
Long-term retainage | 14,228 | 4,152
Total retainage | $ 89,818 | $ 60,380

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