Regulatory assets and liabilities Regulatory Liabilities (Details 2) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	$ 39,837	$ 42,935	$ 36,945
Regulatory Liability, Noncurrent	434,936	447,370	453,760
Total regulatory liabilities	474,773	490,305	490,705
Net regulatory position	(25,884)	(72,908)	(58,725)
Natural gas costs refundable through rate adjustments
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	20,556	23,825	19,194
Electric fuel and purchased power deferral
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	6,171	5,824	2,040
Taxes refundable to customers
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	4,223	3,472	4,309
Regulatory Liability, Noncurrent	232,186	246,034	253,703
Plant removal and decommissioning costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Noncurrent	173,367	173,722	174,595
Pension and postretirement benefits
Regulatory Liabilities [Line Items]
Regulatory Liability, Noncurrent	17,991	18,065	15,190
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	8,887	9,814	11,402
Regulatory Liability, Noncurrent	$ 11,392	$ 9,549	$ 10,272