Operating Segments - Schedule of Segment Information (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting Information [Line Items]
Operating revenue	[1]	$ 87.9	$ 89.3	$ 624.0	$ 605.8
Net income (loss)		(17.7)	(8.8)	28.3	32.2
Operating Segments | Questar Gas
Segment Reporting Information [Line Items]
Operating revenue		87.9	89.3	624.0	605.8
Net income (loss)		(10.0)	(8.8)	36.0	32.2
Operating Segments | Corporate and Other
Segment Reporting Information [Line Items]
Operating revenue		0.0	0.0	0.0	0.0
Net income (loss)		$ (7.7)	$ 0.0	$ (7.7)	$ 0.0

[1] See Note 9 for amounts attributable to affiliates