Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Current regulatory assets	$ 7,033	$ 4,707
Noncurrent regulatory assets	23,992	22,747
Total regulatory assets	31,025	27,454
Regulatory Liabilities [Line Items]
Regulatory liabilities	290,430	246,504
Total regulatory liabilities	290,430	248,043
Fuel recovery [Member]
Regulatory Liabilities [Line Items]
Current regulatory liabilities	0	1,539
Postretirement benefits [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	36,154	34,089
Deferred federal taxes-liability
Regulatory Liabilities [Line Items]
Regulatory liabilities	206,527	206,547
Total regulatory liabilities	23,600	0
Deferred state taxes - liability
Regulatory Liabilities [Line Items]
Total regulatory liabilities	12,100	0
Customer tax refund [Domain]
Regulatory Liabilities [Line Items]
Regulatory liabilities	24,094
Asset retirement obligations, net [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	11,535	5,868
Fuel recovery [Member]
Regulatory Assets [Line Items]
Current regulatory assets	6,619	0
Levelized depreciation [Member]
Regulatory Assets [Line Items]
Current regulatory assets	414	4,707
Noncurrent regulatory assets	18,109	16,149
Deferred taxes on equity funds used during construction [Member]
Regulatory Assets [Line Items]
Noncurrent regulatory assets	$ 5,883	$ 6,598