Operations by Line of Business (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013 lines_of_business	Mar. 31, 2012	Mar. 31, 2013	Mar. 31, 2012
Segment Reporting Information [Line Items]
Number of principal complementary lines of business	3
Revenues from Unaffiliated Customers	$ 476.9	$ 425.7	$ 1,150.1	$ 1,150.1
Revenues from Affiliated Companies	92.6	85.0	358.6	336.0
Operating Income (Loss)	131.4	130.9	376.2	375.0
Net Income (Loss)	72.9	75.2	209.7	213.2
Questar Gas [Member]
Segment Reporting Information [Line Items]
Revenues from Unaffiliated Customers	418.3	366.0	912.0	917.6
Revenues from Affiliated Companies	0.3	0.8	2.0	3.3
Operating Income (Loss)	64.3	61.8	95.4	97.1
Net Income (Loss)	37.0	34.9	49.2	47.6
Wexpro [Member]
Segment Reporting Information [Line Items]
Revenues from Unaffiliated Customers	10.3	9.2	37.2	32.9
Revenues from Affiliated Companies	72.8	65.7	281.2	258.4
Operating Income (Loss)	39.8	37.2	160.7	148.4
Net Income (Loss)	26.3	24.3	105.9	97.2
Questar Pipeline [Member]
Segment Reporting Information [Line Items]
Revenues from Unaffiliated Customers	48.3	50.5	200.9	199.6
Revenues from Affiliated Companies	19.5	18.5	75.4	74.3
Operating Income (Loss)	30.2	31.8	122.4	128.4
Net Income (Loss)	15.8	16.6	63.9	69.2
Questar Corp and Other [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	(2.9)	0.1	(2.3)	1.1
Net Income (Loss)	$ (6.2)	$ (0.6)	$ (9.3)	$ (0.8)