Rate and Regulatory Matters (Details) $ in Thousands				12 Months Ended
	Oct. 01, 2018 $ / dekatherm	Jan. 01, 2018 USD ($) $ / dekatherm	Jan. 23, 2017 USD ($)	Dec. 31, 2018	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Public Utilities, General Disclosures [Line Items]
Duration of effective period of rates					5 years
Annual Cost of Service | $			$ 440,000
Regulatory Liabilities | $					$ 248,043	$ 32,183
Deferred Income Tax Charge [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities | $		$ 23,600
Subsequent Event [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Liability, Amortization Period				5 years
Subsequent Event [Member] | Phase 1 Demand Rate per Dth Pending
Public Utilities, General Disclosures [Line Items]
Demand Rate | $ / dekatherm		0.39294
Subsequent Event [Member] | Phase 1 Commodity Rate Pending
Public Utilities, General Disclosures [Line Items]
Commodity Rate | $ / dekatherm		0.00832
Subsequent Event [Member] | Phase 2 Demand Rate per Dth Pending
Public Utilities, General Disclosures [Line Items]
Demand Rate | $ / dekatherm	0.39033
Subsequent Event [Member] | Phase 2 Commodity Rate Pending
Public Utilities, General Disclosures [Line Items]
Commodity Rate | $ / dekatherm	0.00832
Subsequent Event [Member] | Deferred Income Tax Charge [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities | $		$ 23,600