Regulatory assets and liabilities Regulatory Liabilities (Details 2) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	$ 48,000	$ 42,935	$ 49,974
Regulatory Liability, Noncurrent	438,652	447,370	455,864
Total regulatory liabilities	486,652	490,305	505,838
Net regulatory position	(75,070)	(72,908)	(98,013)
Natural gas costs refundable through rate adjustments
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	29,557	23,825	25,737
Electric fuel and purchased power deferral
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	7,799	5,824	4,686
Taxes refundable to customers
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	4,012	3,472	7,104
Regulatory Liability, Noncurrent	236,142	246,034	256,921
Plant removal and decommissioning costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Noncurrent	173,399	173,722	173,661
Pension and postretirement benefits
Regulatory Liabilities [Line Items]
Regulatory Liability, Noncurrent	18,015	18,065	15,215
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	6,632	9,814	12,447
Regulatory Liability, Noncurrent	$ 11,096	$ 9,549	$ 10,067