Regulatory assets and liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Regulatory assets and liabilities
Regulatory assets:	$ 340,117	$ 300,963
Regulatory liabilities:	521,432	519,298
Net regulatory position	(181,315)	(218,335)
Regulatory assets not earning a rate of return	313,500	269,100
Taxes recoverable from/refundable to customers
Regulatory assets and liabilities
Regulatory liabilities:	277,833	279,668
Plant removal and decommissioning costs
Regulatory assets and liabilities
Regulatory liabilities:	173,143	176,190
Natural gas costs refundable through rate adjustments
Regulatory assets and liabilities
Regulatory liabilities:	29,995	28,514
Pension and postretirement benefits
Regulatory assets and liabilities
Regulatory liabilities:	15,264	16,021
Other regulatory assets/liabilities
Regulatory assets and liabilities
Regulatory liabilities:	$ 25,197	18,905
Pension and postretirement benefits
Regulatory assets and liabilities
Estimated recovery period	(e)
Regulatory assets:	$ 165,898	163,896
Asset retirement obligation costs
Regulatory assets and liabilities
Estimated recovery period	Over plant lives
Regulatory assets:	$ 60,097	56,078
Natural gas costs recoverable through rate adjustments
Regulatory assets and liabilities
Estimated recovery period	Up to 1 year
Regulatory assets:	$ 42,652	14,465
Taxes recoverable from/refundable to customers
Regulatory assets and liabilities
Estimated recovery period	Over plant lives
Regulatory assets:	$ 11,946	12,073
Manufactured gas plant sites remediation
Regulatory assets and liabilities
Estimated recovery period	-
Regulatory assets:	$ 16,504	18,213
Long-term debt refinancing costs
Regulatory assets and liabilities
Estimated recovery period	Up to 19 years
Regulatory assets:	$ 4,898	5,563
Costs related to identifying generation development
Regulatory assets and liabilities
Estimated recovery period	Up to 8 years
Regulatory assets:	$ 2,508	2,960
Other regulatory assets/liabilities
Regulatory assets and liabilities
Estimated recovery period	Up to 20 years
Regulatory assets:	$ 35,614	27,715
Other noncurrent assets | Stranded costs
Regulatory assets and liabilities
Deferred tax liabilities, regulatory assets		15,500
Other noncurrent liabilities | Stranded costs
Regulatory assets and liabilities
Deferred tax liabilities, regulatory assets		$ 270,000