Operations by Line of Business (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014 lines_of_business	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Segment Reporting Information [Line Items]
Number of principal complementary lines of business			3
Revenues by Line of Business	$ 201.3	$ 195.6	$ 658.2	$ 672.5	$ 1,205.7	$ 1,157.3
Operating Income (Loss)	76.5	71.2	224.8	202.6	328.0	374.8
Net Income (Loss)	40.3	39.4	125.4	112.3	174.3	209.9
Questar Gas [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	145.0	140.1	541.3	558.4	968.1	921.9
Operating Income (Loss)	0.3	2.2	70.1	66.5	105.5	96.1
Net Income (Loss)	(3.3)	(1.2)	36.3	35.8	53.3	50.3
Wexpro [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	7.8	8.5	20.1	18.8	46.4	37.6
Operating Income (Loss)	45.8	41.4	94.3	81.2	180.8	162.4
Net Income (Loss)	30.1	28.4	61.9	54.7	117.8	108.5
Questar Pipeline [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	48.1	47.0	96.1	95.3	190.3	197.8
Operating Income (Loss)	30.4	28.1	60.5	58.3	37.7	119.7
Net Income (Loss)	15.7	14.5	31.4	30.3	9.3	62.3
Questar Corp and Other [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	0.4	0	0.7	0	0.9	0
Operating Income (Loss)	0	(0.5)	(0.1)	(3.4)	4.0	(3.4)
Net Income (Loss)	(2.2)	(2.3)	(4.2)	(8.5)	(6.1)	(11.2)
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	112.6	92.1	221.0	184.7	408.4	361.8
Intersegment Eliminations [Member] | Questar Gas [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	0	0.1	0	0.4	0.2	1.1
Intersegment Eliminations [Member] | Wexpro [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	94.9	74.3	184.0	147.1	331.7	285.9
Intersegment Eliminations [Member] | Questar Pipeline [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	$ 17.7	$ 17.7	$ 37.0	$ 37.2	$ 76.5	$ 74.8