Questar Southern Trails Pipeline Eastern Segment Impairment (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Property, Plant and Equipment [Line Items]
Pre-tax asset impairment charge	$ 0	$ 0	$ 80.6	$ 0
Questar Pipeline [Member]
Property, Plant and Equipment [Line Items]
Pre-tax asset impairment charge	0	0	80.6	0
Questar Pipeline [Member] | Questar Southern Trails Pipeline - Eastern Segment [Member]
Property, Plant and Equipment [Line Items]
Pre-tax asset impairment charge			80.6
After-tax asset impairment charge			$ 52.4