Operations by Line of Business (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 lines_of_business	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Segment Reporting Information [Line Items]
Number of principal complementary lines of business			3
Revenues by Line of Business	$ 157.9	$ 149.6	$ 816.1	$ 822.1	$ 1,214.0	$ 1,165.1
Operating Income (Loss)	68.9	(19.4)	293.7	183.2	416.3	291.8
Net Income (Loss)	38.6	(19.2)	164.0	93.1	232.1	156.9
Questar Gas [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	100.7	89.4	642.0	647.8	979.4	928.8
Operating Income (Loss)	(9.4)	(11.2)	60.7	55.3	107.3	95.6
Net Income (Loss)	(9.1)	(9.0)	27.2	26.8	53.2	50.6
Wexpro [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	9.4	13.2	29.5	32.0	42.6	41.0
Operating Income (Loss)	47.2	40.7	141.5	121.9	187.3	162.1
Net Income (Loss)	31.1	27.5	93.0	82.2	121.4	109.6
Questar Pipeline [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	47.3	47.0	143.4	142.3	190.6	195.3
Operating Income (Loss)	28.8	(52.6)	89.3	5.7	119.1	34.5
Net Income (Loss)	14.9	(37.9)	46.3	(7.6)	62.1	7.3
Questar Corp and Other [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	0.5	0	1.2	0	1.4	0
Operating Income (Loss)	2.3	3.7	2.2	0.3	2.6	(0.4)
Net Income (Loss)	1.7	0.2	(2.5)	(8.3)	(4.6)	(10.6)
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	100.0	85.9	321.0	270.6	422.5	359.5
Intersegment Eliminations [Member] | Questar Gas [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	0	0.2	0	0.6	0	0.8
Intersegment Eliminations [Member] | Wexpro [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	82.3	68.1	266.3	215.2	345.9	284.7
Intersegment Eliminations [Member] | Questar Pipeline [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	$ 17.7	$ 17.6	$ 54.7	$ 54.8	$ 76.6	$ 74.0