Regulatory assets and liabilities (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Regulatory Assets and Liabilities
Regulatory Assets	$ 431,591	$ 340,117
Regulatory Liabilities	490,317	521,432
Net Regulatory Position	(58,726)	(181,315)
Regulatory assets not earning a rate of return	278,200	313,500
Taxes recoverable from/refundable to customers
Regulatory Assets and Liabilities
Regulatory Liabilities	258,012	277,833
Plant removal
Regulatory Assets and Liabilities
Regulatory Liabilities	176,016	173,143
Natural gas costs refundable through rate adjustments
Regulatory Assets and Liabilities
Regulatory Liabilities	19,194	29,995
Pension and postretirement benefits
Regulatory Assets and Liabilities
Regulatory Liabilities	12,942	15,264
Other Regulatory Assets/Liabilities
Regulatory Assets and Liabilities
Regulatory Liabilities	$ 24,153	25,197
Pension and postretirement benefits
Regulatory Assets and Liabilities
Estimated Recovery Period	(e)
Regulatory Assets	$ 165,843	165,898
Natural gas costs recoverable through rate adjustments
Regulatory Assets and Liabilities
Estimated Recovery Period	Up to 3 years
Regulatory Assets	$ 93,001	42,652
Asset Retirement Obligation Costs
Regulatory Assets and Liabilities
Estimated Recovery Period	Over plant lives
Regulatory Assets	$ 64,771	60,097
Cost recovery mechanisms
Regulatory Assets and Liabilities
Estimated Recovery Period	Up to 4 years
Regulatory Assets	$ 20,912	17,948
Manufactured gas plant site remediation
Regulatory Assets and Liabilities
Estimated Recovery Period	-
Regulatory Assets	$ 15,739	17,068
Taxes recoverable from/refundable to customers
Regulatory Assets and Liabilities
Estimated Recovery Period	Over plant lives
Regulatory Assets	$ 11,600	11,946
Plant removal
Regulatory Assets and Liabilities
Estimated Recovery Period	-
Regulatory Assets	$ 26,152	0
Conservation programs
Regulatory Assets and Liabilities
Estimated Recovery Period	Up to 1 year
Regulatory Assets	$ 9,467	7,494
Long-term debt refinancing costs
Regulatory Assets and Liabilities
Estimated Recovery Period	Up to 19 years
Regulatory Assets	$ 4,439	4,898
Costs related to identifying generation development
Regulatory Assets and Liabilities
Estimated Recovery Period	Up to 8 years
Regulatory Assets	$ 2,166	2,508
Other Regulatory Assets/Liabilities
Regulatory Assets and Liabilities
Estimated Recovery Period	Up to 20 years
Regulatory Assets	$ 17,501	$ 9,608