Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Regulatory Assets [Line Items]
Current regulatory assets	$ 978	$ 7,033
Regulatory assets, noncurrent	21,935	23,992
Total regulatory assets	22,913	31,025
Regulatory Liabilities [Line Items]
Regulatory liabilities	323,032	290,430
Total regulatory liabilities	327,153	290,430
Fuel recovery
Regulatory Liabilities [Line Items]
Current regulatory liabilities	4,121	0
Postretirement benefits
Regulatory Liabilities [Line Items]
Regulatory liabilities	37,722	36,154
Deferred federal taxes-liability
Regulatory Liabilities [Line Items]
Regulatory liabilities	206,527	206,527
Deferred state taxes - liability
Regulatory Liabilities [Line Items]
Regulatory liabilities	12,120	12,120
Customer tax refund [Domain]
Regulatory Liabilities [Line Items]
Regulatory liabilities	49,598	24,094
Asset retirement obligations, net [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	17,065	11,535
Accrued Interest on Deferred Regulatory Tax Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	2,400
Regulatory charges resulting from tax rate changes [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	23,600
Levelized depreciation
Regulatory Assets [Line Items]
Current regulatory assets	978	414
Regulatory assets, noncurrent	16,437	18,109
Fuel recovery
Regulatory Assets [Line Items]
Current regulatory assets	0	6,619
Grossed-up deferred taxes on equity funds used during construction
Regulatory Assets [Line Items]
Regulatory assets, noncurrent	$ 5,498	$ 5,883