Regulatory assets and liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Regulatory assets and liabilities
Regulatory assets:	$ 417,397	$ 340,117
Regulatory liabilities:	490,305	521,432
Net regulatory position	(72,908)	(181,315)
Regulatory assets not earning a rate of return	276,500	313,500
Taxes recoverable from/refundable to customers
Regulatory assets and liabilities
Regulatory liabilities:	249,506	277,833
Plants to be retired
Regulatory assets and liabilities
Regulatory liabilities:	173,722	173,143
Natural gas costs refundable through rate adjustments
Regulatory assets and liabilities
Regulatory liabilities:	23,825	29,995
Pension and postretirement benefits
Regulatory assets and liabilities
Regulatory liabilities:	18,065	15,264
Other regulatory assets/liabilities
Regulatory assets and liabilities
Regulatory liabilities:	$ 25,187	25,197
Pension and postretirement benefits
Regulatory assets and liabilities
Estimated recovery period	(e)
Regulatory assets:	$ 157,069	165,898
Natural gas costs recoverable through rate adjustments
Regulatory assets and liabilities
Estimated recovery period	Up to 3 years
Regulatory assets:	$ 89,204	42,652
Asset retirement obligation costs
Regulatory assets and liabilities
Estimated recovery period	Over plant lives
Regulatory assets:	$ 66,000	60,097
Plants to be retired
Regulatory assets and liabilities
Estimated recovery period	-
Regulatory assets:	$ 32,931	0
Cost recovery mechanisms (a) (b)
Regulatory assets and liabilities
Estimated recovery period	Up to 3 years
Regulatory assets:	$ 19,396	17,948
Manufactured gas plant sites remediation
Regulatory assets and liabilities
Estimated recovery period	-
Regulatory assets:	$ 15,347	17,068
Taxes recoverable from/refundable to customers
Regulatory assets and liabilities
Estimated recovery period	Over plant lives
Regulatory assets:	$ 11,486	11,946
Conservation programs (a) (b)
Regulatory assets and liabilities
Estimated recovery period	Up to 3 years
Regulatory assets:	$ 7,405	7,494
Long-term debt refinancing costs
Regulatory assets and liabilities
Estimated recovery period	Up to 18 years
Regulatory assets:	$ 4,286	4,898
Costs related to identifying generation development
Regulatory assets and liabilities
Estimated recovery period	Up to 7 years
Regulatory assets:	$ 2,052	2,508
Other regulatory assets/liabilities
Regulatory assets and liabilities
Estimated recovery period	Up to 19 years
Regulatory assets:	$ 12,221	$ 9,608