Regulatory liabilities (Details 2) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2022	Dec. 31, 2021	Jun. 30, 2021
Regulatory Liabilities [Line Items]
Regulatory liabilities due within one year	$ 17,884	$ 16,303	$ 23,454
Regulatory liabilities	433,019	428,790	428,467
Total regulatory liabilities	450,903	445,093	451,921
Net regulatory position	$ (1,307)	31,449	34,573
Electric fuel and purchased power deferral
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of June 30, 2022	Up to 1 year
Regulatory liabilities due within one year	$ 4,161	0	5,431
Taxes refundable to customers
Regulatory Liabilities [Line Items]
Regulatory liabilities due within one year	3,728	3,841	3,434
Regulatory liabilities	$ 209,022	215,421	222,098
Taxes refundable to customers | Minimum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of June 30, 2022	Up to 1 year
Taxes refundable to customers | Maximum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of June 30, 2022	Over plant lives
Cost Recovery Mechanisms
Regulatory Liabilities [Line Items]
Regulatory liabilities due within one year	$ 3,172	214	1,388
Regulatory liabilities	$ 10,898	7,727	2,499
Cost Recovery Mechanisms | Minimum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of June 30, 2022	Up to 1 year
Cost Recovery Mechanisms | Maximum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of June 30, 2022	Up to 20 years
Natural gas costs refundable through rate adjustments
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of June 30, 2022	Up to 1 year
Regulatory liabilities due within one year	$ 738	6,700	8,935
Other regulatory liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities due within one year	6,085	5,548	4,266
Regulatory liabilities	$ 6,649	4,360	3,634
Other regulatory liabilities | Minimum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of June 30, 2022	Up to 1 year
Other regulatory liabilities | Maximum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of June 30, 2022	Up to 16 years
Plant removal and decommissioning costs
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of June 30, 2022	Over plant lives
Regulatory liabilities	$ 172,755	168,152	171,381
Pension and postretirement benefits
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of June 30, 2022	**
Regulatory liabilities	$ 19,686	20,434	16,940
Accumulated Deferred ITC
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of June 30, 2022	Up to 20 years
Regulatory liabilities	$ 14,009	$ 12,696	$ 11,915