Regulatory assets and liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Regulatory assets and liabilities
Current regulatory assets	$ 68,527	$ 63,613
Regulatory assets	379,381	353,784
Regulatory assets:	447,908	417,397
Regulatory liabilities due within one year	31,450	42,935
Regulatory liabilities	428,075	447,370
Regulatory liabilities:	459,525	490,305
Net regulatory position	(11,617)	(72,908)
Regulatory assets not earning a rate of return	$ 332,500	276,500
Natural gas costs refundable through rate adjustments
Regulatory assets and liabilities
Estimated recovery period	Up to 1 year
Regulatory liabilities due within one year	$ 18,565	23,825
Liabilities Electric fuel and purchased power deferral
Regulatory assets and liabilities
Estimated recovery period	Up to 1 year
Regulatory liabilities due within one year	$ 3,667	5,824
Taxes recoverable from/refundable to customers
Regulatory assets and liabilities
Regulatory liabilities due within one year	3,557	3,472
Regulatory liabilities	$ 227,850	246,034
Taxes recoverable from/refundable to customers | Minimum
Regulatory assets and liabilities
Estimated recovery period	Up to 1 year
Taxes recoverable from/refundable to customers | Maximum
Regulatory assets and liabilities
Estimated recovery period	Over plant lives
Plants to be retired
Regulatory assets and liabilities
Estimated recovery period	Over plant lives
Regulatory liabilities	$ 167,171	173,722
Pension and postretirement benefits
Regulatory assets and liabilities
Estimated recovery period	**
Regulatory liabilities	$ 16,989	18,065
Other regulatory assets/liabilities
Regulatory assets and liabilities
Regulatory liabilities due within one year	5,661	9,814
Regulatory liabilities	$ 16,065	9,549
Other regulatory assets/liabilities | Minimum
Regulatory assets and liabilities
Estimated recovery period	Up to 1 year
Other regulatory assets/liabilities | Maximum
Regulatory assets and liabilities
Estimated recovery period	Up to 21 years
Natural gas costs recoverable through rate adjustments
Regulatory assets and liabilities
Current regulatory assets	$ 42,481	42,823
Regulatory assets	$ 21,539	46,381
Natural gas costs recoverable through rate adjustments | Minimum
Regulatory assets and liabilities
Estimated recovery period	Up to 1 year
Natural gas costs recoverable through rate adjustments | Maximum
Regulatory assets and liabilities
Estimated recovery period	Up to 2 years
Cost recovery mechanisms
Regulatory assets and liabilities
Current regulatory assets	$ 10,645	6,288
Regulatory assets	$ 16,245	13,108
Cost recovery mechanisms | Minimum
Regulatory assets and liabilities
Estimated recovery period	Up to 1 year
Cost recovery mechanisms | Maximum
Regulatory assets and liabilities
Estimated recovery period	Up to 10 years
Conservation programs
Regulatory assets and liabilities
Estimated recovery period	Up to 1 year
Current regulatory assets	$ 7,117	6,963
Pension and postretirement benefits
Regulatory assets and liabilities
Estimated recovery period	**
Regulatory assets	$ 155,942	157,069
Asset retirement obligation costs
Regulatory assets and liabilities
Estimated recovery period	Over plant lives
Regulatory assets	$ 71,740	66,000
Plants to be retired
Regulatory assets and liabilities
Estimated recovery period	-
Regulatory assets	$ 65,919	32,931
Manufactured gas plant sites remediation
Regulatory assets and liabilities
Estimated recovery period	-
Regulatory assets	$ 26,429	15,126
Taxes recoverable from/refundable to customers
Regulatory assets and liabilities
Estimated recovery period	Over plant lives
Regulatory assets	$ 10,785	11,486
Long-term debt refinancing costs
Regulatory assets and liabilities
Estimated recovery period	Up to 40 years
Regulatory assets	$ 4,426	4,286
Other regulatory assets/liabilities
Regulatory assets and liabilities
Current regulatory assets	8,284	7,539
Regulatory assets	$ 6,356	$ 7,397
Other regulatory assets/liabilities | Minimum
Regulatory assets and liabilities
Estimated recovery period	Up to 1 year
Other regulatory assets/liabilities | Maximum
Regulatory assets and liabilities
Estimated recovery period	Up to 18 years