Regulatory liabilities (Details 2) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022
Regulatory Liabilities [Line Items]
Regulatory liabilities due within one year	$ 45,013	$ 26,440	$ 18,599
Regulatory liabilities	495,112	448,454	433,212
Total regulatory liabilities	540,125	474,894	451,811
Net regulatory position	$ 58,092	19,857	40,791
Natural gas costs refundable through rate adjustments
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2023	Up to 1 year
Regulatory liabilities due within one year	$ 20,445	955	873
Electric fuel and purchased power deferral
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2023	Up to 1 year
Regulatory liabilities due within one year	$ 0	4,929	3,763
Cost Recovery Mechanisms
Regulatory Liabilities [Line Items]
Regulatory liabilities due within one year	5,754	1,977	2,674
Regulatory liabilities	$ 20,012	14,025	12,535
Cost Recovery Mechanisms | Minimum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2023	Up to 1 year
Cost Recovery Mechanisms | Maximum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2023	Up to 19 years
Conservation programs
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2023	Up to 1 year
Regulatory liabilities due within one year	$ 1,976	4,126	325
Taxes refundable to customers
Regulatory Liabilities [Line Items]
Regulatory liabilities due within one year	1,513	3,937	4,264
Regulatory liabilities	$ 194,804	203,222	205,517
Taxes refundable to customers | Minimum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2023	Up to 1 year
Taxes refundable to customers | Maximum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2023	Over plant lives
Refundable Fuel and Electric Costs
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2023	Up to 1 year
Regulatory liabilities due within one year	$ 18	3,253	1,812
Other regulatory liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities due within one year	15,307	7,263	4,888
Regulatory liabilities	$ 1,751	1,587	6,327
Other regulatory liabilities | Minimum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2023	Up to 1 year
Other regulatory liabilities | Maximum [Member]
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2023	Up to 15 years
Plant removal and decommissioning costs
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2023	Over plant lives
Regulatory liabilities	$ 220,499	208,650	174,481
Accumulated Deferred ITC
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2023	Up to 19 years
Regulatory liabilities	$ 14,800	13,594	14,665
Pension and postretirement benefits
Regulatory Liabilities [Line Items]
Estimated Recovery or Refund Period as of September 30, 2023	**
Regulatory liabilities	$ 7,120	7,376	19,687
Environmental compliance programs
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 36,126	$ 0	$ 0