Operations by Line of Business (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2016 USD ($) lines_of_business	Mar. 31, 2015 USD ($)	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Number of principal complementary lines of business | lines_of_business	3
Revenues by Line of Business	$ 456.3	$ 428.6	$ 1,162.6	$ 1,161.0
Operating Income (Loss)	135.7	147.0	363.1	404.1
Net Income (Loss)	78.5	84.6	202.6	226.0
Questar Gas [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	407.9	374.8	950.7	939.4
Operating Income (Loss)	83.4	76.6	129.4	116.3
Net Income (Loss)	47.6	43.8	68.1	59.4
Wexpro [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	3.5	6.3	20.1	29.6
Operating Income (Loss)	39.7	41.7	145.4	176.5
Net Income (Loss)	26.2	27.7	97.4	118.7
Questar Pipeline [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	43.0	46.8	184.1	189.0
Operating Income (Loss)	27.6	27.4	114.6	114.9
Net Income (Loss)	14.2	13.9	59.9	58.8
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	1.9	0.7	7.7	3.0
Operating Income (Loss)	(15.0)	1.3	(26.3)	(3.6)
Net Income (Loss)	(9.5)	(0.8)	(22.8)	(10.9)
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	99.1	104.2	389.1	419.8
Intersegment Eliminations [Member] | Wexpro [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	79.8	84.8	314.1	346.0
Intersegment Eliminations [Member] | Questar Pipeline [Member]
Segment Reporting Information [Line Items]
Revenues by Line of Business	$ 19.3	$ 19.4	$ 75.0	$ 73.8