Wexpro and Wexpro II Agreements (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Agreement [Line Items]
Percentage of Wexpro retention (in hundredths)	46.00%
Wexpro net investment base at December 31, (in millions)	$ 589.7	$ 531.1	$ 474.4
Average annual rate of return (after tax) (in hundredths)	19.70%	19.90%	20.00%
Oil and NGL income sharing (in millions)	0.6	2.5	3.3
Net cash paid, after post-closing adjustments	$ 104.3
Trail acquisition specified Questar Gas supply percentage (in hundredths)	65.00%
Gas-development drilling [Member]
Agreement [Line Items]
Percentage of Wexpro after tax rate of return (in hundredths)	20.20%
Percentage of premium added to base after-tax rate of return (in hundredths)	8.00%
Natural Gas operations [Member]
Agreement [Line Items]
Percentage of Wexpro after tax rate of return (in hundredths)	12.20%
Developmental Oil Drilling [Member]
Agreement [Line Items]
Percentage of Wexpro after tax rate of return (in hundredths)	17.20%
Percentage of Wexpro retention (in hundredths)	46.00%
Percentage of Questar Gas retention (in hundredths)	54.00%
Percentage of premium added to base after-tax rate of return (in hundredths)	5.00%
Crude Oil Production [Member]
Agreement [Line Items]
Percentage of Wexpro after tax rate of return (in hundredths)	12.20%
Percentage of Wexpro retention (in hundredths)	46.00%