OPERATING SEGMENT - Schedule of Segment Information (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Segment Reporting Information [Line Items]
Operating revenue		$ 297.3	$ 87.9	$ 128.2	$ 407.9	$ 311.8	$ 89.3	$ 141.7	$ 374.8	$ 921.3	[1]	$ 917.6	[1]	$ 960.9	[1]
Depreciation and amortization										61.0		55.1		53.6
Interest income										0.3		1.2		1.7
Interest expense	[1]									30.2		28.3		28.2
Income taxes										37.1		34.8		32.0
Net income (loss)		28.9	$ (17.7)	$ (1.6)	$ 47.6	32.1	$ (8.8)	$ (2.8)	$ 43.8	57.2		64.3		55.2
Capital expenditures										240.4		217.4		174.7
Total assets (billions)		2,517.0				2,192.7				2,517.0		2,192.7
Operating Segments | Questar Gas
Segment Reporting Information [Line Items]
Operating revenue										921.3		917.6		960.9
Depreciation and amortization										61.0		55.1		53.6
Interest income										0.3		1.2		1.7
Interest expense										30.2		28.3		28.2
Income taxes										43.4		34.8		32.0
Net income (loss)										66.8		64.3		55.2
Capital expenditures										240.4		217.4		174.7
Total assets (billions)		2,500.0				2,200.0				2,500.0		2,200.0
Operating Segments | Corporate and Other
Segment Reporting Information [Line Items]
Operating revenue										0.0		0.0		0.0
Depreciation and amortization										0.0		0.0		0.0
Interest income										0.0		0.0		0.0
Interest expense										0.0		0.0		0.0
Income taxes										(6.3)		0.0		0.0
Net income (loss)										(9.6)		0.0		0.0
Capital expenditures										0.0		0.0		$ 0.0
Total assets (billions)		$ 0.0				$ 0.0				$ 0.0		$ 0.0

[1] See Note 18 for amounts attributable to related parties.