Accounts receivable and allowance for doubtful accounts (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
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