Regulatory liabilities (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Regulatory Liabilities [Line Items]
Regulatory liabilities due within one year	$ 16,303	$ 31,450
Regulatory liabilities	428,790	428,075
Total regulatory liabilities	445,093	459,525
Net Regulatory Position	$ 31,449	(11,617)
Natural gas costs refundable through rate adjustments
Regulatory Liabilities [Line Items]
Regulatory liability, refund period	Up to 1 year
Regulatory liabilities due within one year	$ 6,700	18,565
Taxes refundable to customers
Regulatory Liabilities [Line Items]
Regulatory liabilities due within one year	3,841	3,557
Regulatory liabilities	$ 215,421	227,850
Taxes refundable to customers | Minimum
Regulatory Liabilities [Line Items]
Regulatory liability, refund period	Up to 1 year
Taxes refundable to customers | Maximum
Regulatory Liabilities [Line Items]
Regulatory liability, refund period	Over plant lives
Electric fuel and purchased power deferral
Regulatory Liabilities [Line Items]
Regulatory liability, refund period	Up to 1 year
Regulatory liabilities due within one year	$ 0	3,667
Other regulatory liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities due within one year	5,762	5,661
Regulatory liabilities	$ 24,783	16,065
Other regulatory liabilities | Minimum
Regulatory Liabilities [Line Items]
Regulatory liability, refund period	Up to 1 year
Other regulatory liabilities | Maximum
Regulatory Liabilities [Line Items]
Regulatory liability, refund period	Up to 20 years
Plant removal and decommissioning costs
Regulatory Liabilities [Line Items]
Regulatory liability, refund period	Over plant lives
Regulatory liabilities	$ 168,152	167,171
Pension and postretirement benefits
Regulatory Liabilities [Line Items]
Regulatory liability, refund period	**
Regulatory liabilities	$ 20,434	$ 16,989