Employee Benefits Subsidiary plan participation and 401(k) Retirement Income Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Percentage of employees' qualifying earnings to which the employer matching contribution applies (in hundredths)	100.00%
Percentage of employees' qualifying earnings matched by the employer for pre-tax 401(k) Plan purchases (in hundredths)	6.00%
Percentage of employees' qualifying prior year earnings contributed by the employer on a non-matching basis for pre-tax 401(k) Plan purchases (in hundredths)	4.00%
Yearly 401(k) Plan contribution expense amount recognized in the current period	$ 9.8	$ 9.1	$ 7.2
Pension Plans, Defined Benefit [Member]
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Net periodic cost	37.5	17.6	36.4
Other Postretirement Benefit Plans, Defined Benefit [Member]
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Net periodic cost	3.1	2.9	5.3
Questar Gas [Member]
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Yearly 401(k) Plan contribution expense amount recognized in the current period	4.5	4.1	3.4
Questar Gas [Member] | Pension Plans, Defined Benefit [Member]
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Net periodic cost	10.4	8.5	18.1
Share of total defined benefit plan contributions	34.9	21.8	29.6
Questar Gas [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Net periodic cost	0.9	0.8	2.4
Share of total defined benefit plan contributions	0.5	1.1	2.0
Questar Pipeline [Member]
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Yearly 401(k) Plan contribution expense amount recognized in the current period	1.8	1.8	1.4
Questar Pipeline [Member] | Pension Plans, Defined Benefit [Member]
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Net periodic cost	3.8	3.2	6.7
Share of total defined benefit plan contributions	13.4	8.1	11.2
Questar Pipeline [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Net periodic cost	(0.3)	(0.4)	0.2
Share of total defined benefit plan contributions	$ 0.1	$ 0.4	$ 0.7