Asset Retirement Obligations (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
ARO liability beginning balance	$ 63.8	$ 60.9
Accretion	3.4	3.0
Liabilities incurred	3.1	1.5
Revisions in estimated cash flows	(1.7)	0.8
Liabilities settled	(1.4)	(2.4)
ARO liability ending balance	67.2	63.8
Asset Retirement Obligations By Line Of Business [Abstract]
AROs by line of business	67.2	63.8
Questar Gas [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
ARO liability ending balance	0.5	0.7
Asset Retirement Obligations By Line Of Business [Abstract]
AROs by line of business	0.5	0.7
Wexpro [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
ARO liability ending balance	64.6	60.3
Asset Retirement Obligations By Line Of Business [Abstract]
AROs by line of business	64.6	60.3
Questar Pipeline [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
ARO liability ending balance	2.1	2.8
Asset Retirement Obligations By Line Of Business [Abstract]
AROs by line of business	$ 2.1	$ 2.8