Regulatory liabilities (Details 2) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Regulatory Liabilities [Line Items]
Regulatory liabilities due within one year	$ 16,491	$ 31,450	$ 39,837
Regulatory liabilities	426,141	428,075	434,936
Total regulatory liabilities	442,632	459,525	474,773
Net regulatory position	69,748	(11,617)	(25,884)
Taxes refundable to customers
Regulatory Liabilities [Line Items]
Regulatory liabilities due within one year	3,867	3,557	4,223
Regulatory liabilities	$ 218,566	227,850	232,186
Taxes refundable to customers | Minimum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2021	Up to 1 year
Taxes refundable to customers | Maximum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2021	Over plant lives
Electric fuel and purchased power deferral
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2021	Up to 1 year
Regulatory liabilities due within one year	$ 3,205	3,667	6,171
Natural gas costs refundable through rate adjustments
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2021	-
Regulatory liabilities due within one year	$ 0	18,565	20,556
Other regulatory liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities due within one year	9,419	5,661	8,887
Regulatory liabilities	$ 17,977	16,065	11,392
Other regulatory liabilities | Minimum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2021	Up to 1 year
Other regulatory liabilities | Maximum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2021	Up to 21 years
Plant removal and decommissioning costs
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2021	Over plant lives
Regulatory liabilities	$ 172,683	167,171	173,367
Pension and postretirement benefits
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2021	**
Regulatory liabilities	$ 16,915	$ 16,989	$ 17,991