Operations by Line of Business (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015 lines_of_business	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information [Line Items]
Number of principal complementary lines of business	3
Revenues by Line of Business	$ 428.6	$ 456.9	$ 1,161.0	$ 1,200.0
Operating Income (Loss)	147.0	148.3	404.1	322.7
Net Income (Loss)	84.6	85.1	226.0	173.4
Questar Gas [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	374.8	396.3	939.4	963.2
Operating Income (Loss)	76.6	69.8	116.3	107.4
Net Income (Loss)	43.8	39.6	59.4	55.4
Wexpro [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	6.3	12.3	29.6	47.1
Operating Income (Loss)	41.7	48.5	176.5	176.4
Net Income (Loss)	27.7	31.8	118.7	116.1
Questar Pipeline [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	46.8	48.0	189.0	189.2
Operating Income (Loss)	27.4	30.1	114.9	35.4
Net Income (Loss)	13.9	15.7	58.8	8.1
Questar Corp and Other [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	0.7	0.3	3.0	0.5
Operating Income (Loss)	1.3	(0.1)	(3.6)	3.5
Net Income (Loss)	(0.8)	(2.0)	(10.9)	(6.2)
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	104.2	108.4	419.8	387.9
Intersegment Eliminations [Member] | Questar Gas [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	0	0	0	0.3
Intersegment Eliminations [Member] | Wexpro [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	84.8	89.1	346.0	311.1
Intersegment Eliminations [Member] | Questar Pipeline [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	$ 19.4	$ 19.3	$ 73.8	$ 76.5