Operations by Line of Business (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 lines_of_business	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]
Number of principal complementary lines of business			3
Revenues from Unaffiliated Customers	$ 149.6	$ 141.8	$ 822.1	$ 755.9	$ 1,165.1	$ 1,110.7
Operating Income (Loss)	(19.4)	63.6	183.2	267.1	291.8	373.7
Net Income (Loss)	(19.2)	33.8	93.1	148.2	156.9	209.8
Questar Gas [Member]
Segment Reporting Information [Line Items]
Revenues from Unaffiliated Customers	89.4	82.5	647.8	578.7	928.8	876.5
Operating Income (Loss)	(11.2)	(10.7)	55.3	52.6	95.6	90.1
Net Income (Loss)	(9.0)	(9.3)	26.8	23.3	50.6	43.5
Wexpro [Member]
Segment Reporting Information [Line Items]
Revenues from Unaffiliated Customers	13.2	9.8	32.0	27.1	41.0	35.5
Operating Income (Loss)	40.7	41.0	121.9	117.9	162.1	155.3
Net Income (Loss)	27.5	26.4	82.2	76.5	109.6	100.1
Questar Pipeline [Member]
Segment Reporting Information [Line Items]
Revenues from Unaffiliated Customers	47.0	49.5	142.3	150.1	195.3	198.7
Operating Income (Loss)	(52.6)	32.6	5.7	95.2	34.5	126.2
Net Income (Loss)	(37.9)	17.1	(7.6)	49.8	7.3	67.0
Questar Corp and Other [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	3.7	0.7	0.3	1.4	(0.4)	2.1
Net Income (Loss)	0.2	(0.4)	(8.3)	(1.4)	(10.6)	(0.8)
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues from Unaffiliated Customers	85.9	88.2	270.6	262.1	359.5	349.0
Intersegment Eliminations [Member] | Questar Gas [Member]
Segment Reporting Information [Line Items]
Revenues from Unaffiliated Customers	0.2	0.5	0.6	2.3	0.8	3.4
Intersegment Eliminations [Member] | Wexpro [Member]
Segment Reporting Information [Line Items]
Revenues from Unaffiliated Customers	68.1	69.3	215.2	204.6	284.7	271.8
Intersegment Eliminations [Member] | Questar Pipeline [Member]
Segment Reporting Information [Line Items]
Revenues from Unaffiliated Customers	$ 17.6	$ 18.4	$ 54.8	$ 55.2	$ 74.0	$ 73.8