Accounts receivable and allowance for doubtful accounts (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
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