Income Taxes - Narrative (Details) - USD ($) $ in Millions		9 Months Ended		12 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Taxes [Line Items]
U.S. statutory rate		21.00%	35.00%	35.00%
Regulatory liabilities		$ 40.9		$ 9.9	[1]
CET
Income Taxes [Line Items]
Regulatory liabilities	[2]	$ 7.4		$ 4.4

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

[2] 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.