Operations by Line of Business (Details) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) lines_of_business	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)
Segment Reporting Information [Line Items]
Number of principal complementary lines of business | lines_of_business			3
Revenues by Line of Business	$ 199.3	$ 201.3	$ 627.9	$ 658.2	$ 1,159.0	$ 1,205.7
Operating Income (Loss)	75.3	76.5	222.3	224.8	402.9	328.0
Net Income (Loss)	40.6	40.3	125.2	125.4	226.3	174.3
Questar Gas [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	141.7	145.0	516.5	541.3	936.1	968.1
Operating Income (Loss)	1.4	0.3	78.0	70.1	117.4	105.5
Net Income (Loss)	(2.8)	(3.3)	41.0	36.3	59.9	53.3
Wexpro [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	10.1	7.8	16.4	20.1	31.9	46.4
Operating Income (Loss)	41.8	45.8	83.5	94.3	172.5	180.8
Net Income (Loss)	27.9	30.1	55.6	61.9	116.5	117.8
Questar Pipeline [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	46.5	48.1	93.3	96.1	187.4	190.3
Operating Income (Loss)	29.3	30.4	56.7	60.5	113.8	37.7
Net Income (Loss)	15.2	15.7	29.1	31.4	58.3	9.3
Questar Corp and Other [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	1.0	0.4	1.7	0.7	3.6	0.9
Operating Income (Loss)	2.8	0.0	4.1	(0.1)	(0.8)	4.0
Net Income (Loss)	0.3	(2.2)	(0.5)	(4.2)	(8.4)	(6.1)
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	93.4	112.6	197.6	221.0	400.6	408.4
Intersegment Eliminations [Member] | Questar Gas [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	0.0	0.0	0.0	0.0	0.0	0.2
Intersegment Eliminations [Member] | Wexpro [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	75.6	94.9	160.4	184.0	326.7	331.7
Intersegment Eliminations [Member] | Questar Pipeline [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	$ 17.8	$ 17.7	$ 37.2	$ 37.0	$ 73.9	$ 76.5