Asset Retirement Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
AROs at beginning of year	$ 69.3	$ 67.7
Accretion	3.3	3.4
Liabilities incurred	1.9	3.3
Revisions in estimated cash flows	1.8	(2.7)
Liabilities settled	(8.7)	(2.4)
AROs at end of year	67.6	69.3
Asset Retirement Obligations By Line Of Business [Abstract]
AROs by line of business	69.3	67.7
Questar Gas [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
AROs at beginning of year	0.6
AROs at end of year	0.6	0.6
Asset Retirement Obligations By Line Of Business [Abstract]
AROs by line of business	0.6	0.6
Wexpro [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
AROs at beginning of year	66.3
AROs at end of year	64.6	66.3
Asset Retirement Obligations By Line Of Business [Abstract]
AROs by line of business	66.3	66.3
Questar Pipeline [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
AROs at beginning of year	2.3
AROs at end of year	2.3	2.3
Asset Retirement Obligations By Line Of Business [Abstract]
AROs by line of business	2.3	2.3
Questar Corp [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
AROs at beginning of year	0.1
AROs at end of year	0.1	0.1
Asset Retirement Obligations By Line Of Business [Abstract]
AROs by line of business	$ 0.1	$ 0.1