Regulatory liabilities (Details 2) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021
Regulatory Liabilities [Line Items]
Regulatory liabilities due within one year	$ 31,186	$ 16,303	$ 34,266
Regulatory liabilities	433,628	428,790	429,060
Total regulatory liabilities	464,814	445,093	463,326
Net regulatory position	$ (16,508)	31,449	15,043
Natural gas costs refundable through rate adjustments
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of March 31, 2022	Up to 1 year
Regulatory liabilities due within one year	$ 13,107	6,700	16,344
Taxes refundable to customers
Regulatory Liabilities [Line Items]
Regulatory liabilities due within one year	3,470	3,841	3,092
Regulatory liabilities	$ 212,472	215,421	224,795
Taxes refundable to customers | Minimum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of March 31, 2022	Up to 1 year
Taxes refundable to customers | Maximum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of March 31, 2022	Over plant lives
Electric fuel and purchased power deferral
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of March 31, 2022	Up to 1 year
Regulatory liabilities due within one year	$ 0	0	2,001
Other regulatory liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities due within one year	14,609	5,762	12,829
Regulatory liabilities	$ 15,885	12,087	6,442
Other regulatory liabilities | Minimum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of March 31, 2022	Up to 1 year
Other regulatory liabilities | Maximum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of March 31, 2022	Up to 20 years
Plant removal and decommissioning costs
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of March 31, 2022	Over plant lives
Regulatory liabilities	$ 171,485	168,152	169,430
Pension and postretirement benefits
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of March 31, 2022	**
Regulatory liabilities	$ 20,434	20,434	16,965
Accumulated Deferred ITC
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of March 31, 2022	Up to 20 years
Regulatory liabilities	$ 13,352	$ 12,696	$ 11,428