Regulatory assets and liabilities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Feb. 28, 2021	Dec. 31, 2020	Mar. 31, 2020
Regulatory Assets and Liabilities
Regulatory Assets, Current	$ 98,452		$ 68,527	$ 55,206
Regulatory Assets, Noncurrent	379,917		379,381	348,189
Total regulatory assets	478,369		447,908	403,395
Regulatory Liability, Current	34,266		31,450	60,072
Regulatory Liability, Noncurrent	429,060		428,075	443,205
Total regulatory liabilities	463,326		459,525	503,277
Net regulatory position	15,043		(11,617)	(99,882)
Regulatory assets not earning a rate of return	$ 361,400		332,500	285,800
Natural gas costs refundable through rate adjustments
Regulatory Assets and Liabilities
Estimated Recovery or Refund Period as of March 31, 2021	Up to 1 year
Regulatory Liability, Current	$ 16,344		18,565	32,181
Taxes refundable to customers
Regulatory Assets and Liabilities
Regulatory Liability, Current	3,092		3,557	4,002
Regulatory Liability, Noncurrent	$ 224,795		227,850	240,148
Taxes refundable to customers | Minimum [Member]
Regulatory Assets and Liabilities
Estimated Recovery or Refund Period as of March 31, 2021	Up to 1 year
Taxes refundable to customers | Maximum [Member]
Regulatory Assets and Liabilities
Estimated Recovery or Refund Period as of March 31, 2021	Over plant lives
Electric fuel and purchased power deferral
Regulatory Assets and Liabilities
Estimated Recovery or Refund Period as of March 31, 2021	Up to 1 year
Regulatory Liability, Current	$ 2,001		3,667	7,188
Plant removal and decommissioning costs
Regulatory Assets and Liabilities
Estimated Recovery or Refund Period as of March 31, 2021	Over plant lives
Regulatory Liability, Noncurrent	$ 169,430		167,171	174,120
Pension and postretirement benefits
Regulatory Assets and Liabilities
Estimated Recovery or Refund Period as of March 31, 2021	**
Regulatory Liability, Noncurrent	$ 16,965		16,989	18,040
Other Regulatory Liabilities
Regulatory Assets and Liabilities
Regulatory Liability, Current	12,829		5,661	16,701
Regulatory Liability, Noncurrent	$ 17,870		16,065	10,897
Other Regulatory Liabilities | Minimum [Member]
Regulatory Assets and Liabilities
Estimated Recovery or Refund Period as of March 31, 2021	Up to 1 year
Other Regulatory Liabilities | Maximum [Member]
Regulatory Assets and Liabilities
Estimated Recovery or Refund Period as of March 31, 2021	Up to 21 years
Natural gas costs recoverable through rate adjustments
Regulatory Assets and Liabilities
Regulatory Assets, Current	$ 76,782		42,481	39,805
Regulatory Assets, Noncurrent	$ 15,158		21,539	33,027
Natural gas costs recoverable through rate adjustments | Minimum [Member]
Regulatory Assets and Liabilities
Estimated Recovery or Refund Period as of March 31, 2021	Up to 1 year
Natural gas costs recoverable through rate adjustments | Maximum [Member]
Regulatory Assets and Liabilities
Estimated Recovery or Refund Period as of March 31, 2021	Up to 3 years
Cost recovery mechanisms
Regulatory Assets and Liabilities
Regulatory Assets, Current	$ 7,130		10,645	6,686
Regulatory Assets, Noncurrent	$ 15,508		16,245	11,891
Cost recovery mechanisms | Minimum [Member]
Regulatory Assets and Liabilities
Estimated Recovery or Refund Period as of March 31, 2021	Up to 1 year
Cost recovery mechanisms | Maximum [Member]
Regulatory Assets and Liabilities
Estimated Recovery or Refund Period as of March 31, 2021	Up to 10 years
Conservation programs
Regulatory Assets and Liabilities
Estimated Recovery or Refund Period as of March 31, 2021	Up to 1 year
Regulatory Assets, Current	$ 6,679		7,117	6,303
Pension and postretirement benefits
Regulatory Assets and Liabilities
Estimated Recovery or Refund Period as of March 31, 2021	**
Regulatory Assets, Noncurrent	$ 155,924		155,942	157,051
Plant removal and decommissioning costs
Regulatory Assets and Liabilities
Estimated Recovery or Refund Period as of March 31, 2021	-
Regulatory Assets, Noncurrent	$ 73,498		65,919	41,080
Plant costs/asset retirement obligations
Regulatory Assets and Liabilities
Estimated Recovery or Refund Period as of March 31, 2021	Over plant lives
Regulatory Assets, Noncurrent	$ 72,250		71,740	67,475
Manufactured gas plant site remediation
Regulatory Assets and Liabilities
Estimated Recovery or Refund Period as of March 31, 2021	-
Regulatory Assets, Noncurrent	$ 26,002		26,429	15,699
Taxes refundable to customers
Regulatory Assets and Liabilities
Estimated Recovery or Refund Period as of March 31, 2021	Over plant lives
Regulatory Assets, Noncurrent	$ 10,800		10,785	11,082
Long-term debt refinancing costs
Regulatory Assets and Liabilities
Estimated Recovery or Refund Period as of March 31, 2021	Up to 39 years
Regulatory Assets, Noncurrent	$ 4,268		4,426	4,133
Other Regulatory Liabilities
Regulatory Assets and Liabilities
Regulatory Assets, Current	7,861		8,284	2,412
Regulatory Assets, Noncurrent	$ 6,509		$ 6,356	$ 6,751
Other Regulatory Liabilities | Minimum [Member]
Regulatory Assets and Liabilities
Estimated Recovery or Refund Period as of March 31, 2021	Up to 1 year
Other Regulatory Liabilities | Maximum [Member]
Regulatory Assets and Liabilities
Estimated Recovery or Refund Period as of March 31, 2021	Up to 18 years
Increased Gas Costs
Regulatory Assets and Liabilities
Total regulatory assets		$ 44,000