Regulatory Assets and Liabilities - Regulatory Liabilities (Details) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017
Regulatory Liabilities [Line Items]
Regulatory liabilities-current		$ 40.9	$ 9.9	[1]
Regulatory liabilities-noncurrent		453.3	441.0	[1]
Total regulatory liabilities		494.2	450.9
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities-current		2.9	1.3
Regulatory liabilities-noncurrent		4.5	2.1
Purchased-gas adjustment
Regulatory Liabilities [Line Items]
Regulatory liabilities-current	[2]	16.8
CET
Regulatory Liabilities [Line Items]
Regulatory liabilities-current	[3]	7.4	4.4
Cost of plant removal and ARO's
Regulatory Liabilities [Line Items]
Regulatory liabilities-current	[4]	4.2	4.2
Regulatory liabilities-noncurrent	[4]	195.5	194.0
Income taxes refundable through future rates
Regulatory Liabilities [Line Items]
Regulatory liabilities-noncurrent	[5]	253.3	$ 244.9
Cost-of-service impact of 2017 Tax Reform Act
Regulatory Liabilities [Line Items]
Regulatory liabilities-noncurrent	[6]	$ 9.6

[1] Questar Gas’ Balance Sheet at December 31, 2017 has been derived from the audited Balance Sheet at that date.

[2] Purchased gas costs that are different from those provided for in present rates are accumulated and recovered or credited through future rate changes.

[3] The CET represents the difference between actual and allowed revenues. Any deficiency or excess in amounts collected is recovered or refunded through periodic rate adjustments.

[4] Cost of plant removal and AROs represent amounts recovered from customers for costs of future activities to remove assets that are expected to be incurred at the time of retirement.

[5] Amounts recorded to pass the effect of reduced income tax rates from the 2017 Tax Reform Act to customers in future periods, which will reverse at the weighted-average tax rate that was used to build the reserves over the remaining book life of the property.

[6] Balance refundable to customers related to the decrease in revenue requirements for recovery of income taxes. See Note 8 for more information.