Rate and Regulatory Matters (Details) $ in Thousands				12 Months Ended
	Oct. 01, 2018 $ / dekatherm	Jan. 01, 2018 $ / dekatherm	Jan. 23, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Public Utilities, General Disclosures [Line Items]
Annual Cost of Service			$ 440,000
Regulatory Liabilities				$ 290,430	$ 248,043
Phase 1 Demand Rate per Dth Pending
Public Utilities, General Disclosures [Line Items]
Demand Rate | $ / dekatherm		0.39294
Phase 1 Commodity Rate Pending
Public Utilities, General Disclosures [Line Items]
Commodity Rate | $ / dekatherm		0.00832
Phase 2 Demand Rate per Dth Pending
Public Utilities, General Disclosures [Line Items]
Demand Rate | $ / dekatherm	0.39033
Phase 2 Commodity Rate Pending
Public Utilities, General Disclosures [Line Items]
Commodity Rate | $ / dekatherm	0.00832
Deferred Income Tax Charge [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities				$ 23,600	0
Regulatory Liability, Amortization Period				5 years
Accrued Interest on Deferred Regulatory Tax Liability [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities				$ 500
State Tax [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities				$ 12,100	$ 0