Operations by Line of Business (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 lines_of_business	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Segment Reporting Information [Line Items]
Number of principal complementary lines of business			3
Revenues from Unaffiliated Customers	$ 195.6	$ 188.4	$ 672.5	$ 614.1	$ 1,157.3	$ 1,115.6
Revenues from Affiliated Companies	92.1	88.9	184.7	173.9	361.8	344.1
Operating Income (Loss)	71.2	72.6	202.6	203.5	374.8	373.9
Net Income (Loss)	39.4	39.2	112.3	114.4	209.9	212.1
Questar Gas [Member]
Segment Reporting Information [Line Items]
Revenues from Unaffiliated Customers	140.1	130.2	558.4	496.2	921.9	883.8
Revenues from Affiliated Companies	0.1	1.0	0.4	1.8	1.1	3.3
Operating Income (Loss)	2.2	1.5	66.5	63.3	96.1	92.9
Net Income (Loss)	(1.2)	(2.3)	35.8	32.6	50.3	44.9
Wexpro [Member]
Segment Reporting Information [Line Items]
Revenues from Unaffiliated Customers	8.5	8.1	18.8	17.3	37.6	32.6
Revenues from Affiliated Companies	74.3	69.6	147.1	135.3	285.9	266.9
Operating Income (Loss)	41.4	39.7	81.2	76.9	162.4	152.0
Net Income (Loss)	28.4	25.8	54.7	50.1	108.5	99.3
Questar Pipeline [Member]
Segment Reporting Information [Line Items]
Revenues from Unaffiliated Customers	47.0	50.1	95.3	100.6	197.8	199.2
Revenues from Affiliated Companies	17.7	18.3	37.2	36.8	74.8	73.9
Operating Income (Loss)	28.1	30.8	58.3	62.6	119.7	127.5
Net Income (Loss)	14.5	16.1	30.3	32.7	62.3	68.7
Questar Corp and Other [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	(0.5)	0.6	(3.4)	0.7	(3.4)	1.5
Net Income (Loss)	$ (2.3)	$ (0.4)	$ (8.5)	$ (1.0)	$ (11.2)	$ (0.8)