Regulatory Matters (Details) $ in Millions	1 Months Ended		3 Months Ended	9 Months Ended
	Oct. 31, 2018 USD ($)	May 31, 2018 USD ($)	Jun. 30, 2018 USD ($) regulatory_commission MMcf	Sep. 30, 2018 USD ($)
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 increase (decrease) in commodity costs or cost-of-service component of rates reduces due to impacts of 2017 Tax Reform Act		$ (14.5)
Public utilities contingent payable to customers			$ 9.5
Public utilities Liquefaction rate | MMcf			8.2
Annual increase in customer rates				$ 3.5
Wyoming Commission | Subsequent Event
Public Utilities General Disclosures [Line Items]
Public utilities contingent payable to customers	$ 0.5
Utah and Wyoming Commissions
Public Utilities General Disclosures [Line Items]
Gas or non-gas cost increase (decrease) in commodity costs or cost-of-service component of rates reduces due to impacts of 2017 Tax Reform Act		$ (86.0)
Utah and Wyoming Commissions | Subsequent Event
Public Utilities General Disclosures [Line Items]
Gas or non-gas cost increase (decrease) in commodity costs or cost-of-service component of rates reduces due to impacts of 2017 Tax Reform Act	(47.6)
Utah and Wyoming Commissions | CET | Subsequent Event
Public Utilities General Disclosures [Line Items]
Non-gas cost increase (decrease) related to CET or EEP	(9.3)
Utah and Wyoming Commissions | EEP | Subsequent Event
Public Utilities General Disclosures [Line Items]
Non-gas cost increase (decrease) related to CET or EEP	$ 5.9