Rate and Regulatory Matters (Details) $ in Millions				6 Months Ended
	Oct. 01, 2018 $ / dekatherm	Jan. 01, 2018 $ / dekatherm	Jan. 23, 2017 USD ($)	Jun. 30, 2018 USD ($)
Public Utilities, General Disclosures [Line Items]
Annual Cost of Service Pending | $			$ 440.0
Regulatory Liability, Amortization Period				5 years
Phase 1 Demand Rate per Dth Pending [Member]
Public Utilities, General Disclosures [Line Items]
Demand Rate Pending		0.39294
Phase 1 Commodity Rate Pending [Member]
Public Utilities, General Disclosures [Line Items]
Commodity Rate Pending		0.00832
Subsequent Event [Member] | Phase 2 Demand Rate per Dth Pending [Member]
Public Utilities, General Disclosures [Line Items]
Demand Rate Pending	0.39033
Subsequent Event [Member] | Phase 2 Commodity Rate Pending [Member]
Public Utilities, General Disclosures [Line Items]
Commodity Rate Pending	0.00832
Deferred Income Tax Charge [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities | $				$ 11.8