Regulatory Assets and Liabilities - Regulatory Liabilities (Details) - USD ($) $ in Millions		Sep. 30, 2017	Dec. 31, 2016
Regulatory Liabilities [Line Items]
Regulatory liabilities	[1]	$ 5.3	$ 6.5
Regulatory liabilities, noncurrent		193.1	189.1	[2]
Total regulatory liabilities		198.4	195.6
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities		0.3	0.1
Regulatory liabilities, noncurrent		0.2
Curtailment penalty
Regulatory Liabilities [Line Items]
Regulatory liabilities	[3]	0.9
CET
Regulatory Liabilities [Line Items]
Regulatory liabilities	[4]	0.3	2.9
Cost of plant removal and ARO's
Regulatory Liabilities [Line Items]
Regulatory liabilities	[5]	3.8	3.5
Regulatory liabilities, noncurrent	[5]	$ 192.9	$ 189.1

[1] Current regulatory liabilities are presented in other current liabilities in the Balance Sheets.

[2] Questar Gas' Balance Sheet at December 31, 2016 has been derived from the audited Balance Sheet at that date.

[3] Interruptible customers who fail to interrupt service when called upon by Questar Gas incur a curtailment penalty. Penalties recovered by Questar Gas are credited to ratepayers as a reduction to the infrastructure-replacement program.

[4] Represents the difference between actual and allowed revenues. Any deficiency or excess in amounts collected is recovered or returned through periodic rate adjustments.

[5] 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.