Contingencies, Commitments and Leases Commitments (Details) - Questar Gas [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Long-term Purchase Commitment [Line Items]
Natural gas purchased under agreements	$ 82.4	$ 135.8	$ 186.5
Gas purchase agreements [Member]
Long-term Purchase Commitment [Line Items]
2016	22.5
2017	13.6
2018	15.9
2019	15.9
2020	15.9
Transportation and storage commitments - Related party [Member]
Long-term Purchase Commitment [Line Items]
2016	70.5
2017	43.2
2018	13.4
2019	5.2
2020	2.4
Transportation and gathering commitments - Third party [Member]
Long-term Purchase Commitment [Line Items]
2016	31.0
2017	31.0
2018	29.1
2019	26.8
2020	$ 26.8