Regulatory Matters (Details) $ in Millions	1 Months Ended
	Oct. 31, 2017 USD ($)	Sep. 30, 2017 USD ($) regulatory_commission
Public Utilities, General Disclosures [Line Items]
Number of regulatory commissions | regulatory_commission		2
Utah Commission
Public Utilities, General Disclosures [Line Items]
Gas or non-gas cost increases in commodity, transportation costs or infrastructure replacement costs		$ 5.9
Utah and Wyoming Commissions | Subsequent Event
Public Utilities, General Disclosures [Line Items]
Gas or non-gas cost increases in commodity, transportation costs or infrastructure replacement costs	$ 25.8