Operations by Line of Business (Details) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) lines_of_business	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)
Segment Reporting Information [Line Items]
Number of principal complementary lines of business | lines_of_business			3
Revenues by Line of Business	$ 174.1	$ 199.3	$ 630.4	$ 627.9	$ 1,137.4	$ 1,159.0
Operating Income (Loss)	64.4	75.3	200.1	222.3	352.2	402.9
Net Income (Loss)	33.1	40.6	111.6	125.2	195.1	226.3
Questar Gas [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	128.2	141.7	536.1	516.5	937.2	936.1
Operating Income (Loss)	3.8	1.4	87.2	78.0	131.8	117.4
Net Income (Loss)	(1.6)	(2.8)	46.0	41.0	69.3	59.9
Wexpro [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	1.9	10.1	5.4	16.4	11.9	31.9
Operating Income (Loss)	39.1	41.8	78.8	83.5	142.7	172.5
Net Income (Loss)	26.1	27.9	52.3	55.6	95.6	116.5
Questar Pipeline [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	42.9	46.5	85.9	93.3	180.5	187.4
Operating Income (Loss)	26.4	29.3	54.0	56.7	111.7	113.8
Net Income (Loss)	13.6	15.2	27.8	29.1	58.3	58.3
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	1.1	1.0	3.0	1.7	7.8	3.6
Operating Income (Loss)	(4.9)	2.8	(19.9)	4.1	(34.0)	(0.8)
Net Income (Loss)	(5.0)	0.3	(14.5)	(0.5)	(28.1)	(8.4)
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	96.4	93.4	195.5	197.6	392.1	400.6
Intersegment Eliminations [Member] | Wexpro [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	78.9	75.6	158.7	160.4	317.4	326.7
Intersegment Eliminations [Member] | Questar Pipeline [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	$ 17.5	$ 17.8	$ 36.8	$ 37.2	$ 74.7	$ 73.9