Share-Based Compensation Restricted stock units (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	$ 11.4	$ 12.4	$ 10.2
Share-based awards other than options, additional disclosures [Abstract]
Tax benefits from share-based compensation	$ 0.6	$ 2.2	14.0
Restricted Stock Units [Member]
Transactions under the terms of the LTSIP - shares [Roll Forward]
Balance at December 31, 2014	526,102
Granted	351,282
Vested	(294,515)
Forfeited	(32,467)
Balance at December 31, 2015	550,402	526,102
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2014	$ 23.60
Granted	24.38
Vested	23.68
Forfeited	24.32
Balance at December 31, 2015	$ 24.01	$ 23.60
Share-based awards other than options, additional disclosures [Abstract]
Weighted-average remaining vesting period of share-based awards issued (in months)	10 months
Number of shares of common stock to be issued for each vested restricted stock unit (RSU)	1
Vesting period for share-based award (in years)	3 years
Deferred Restricted Stock Units (RSUs) [Member]
Transactions under the terms of the LTSIP - shares [Roll Forward]
Balance at December 31, 2015	47,325
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2015	$ 23.91
Share-based awards other than options, additional disclosures [Abstract]
Weighted-average remaining vesting period of share-based awards issued (in months)	2 months
Number of shares of common stock to be issued for each vested restricted stock unit (RSU)	1
Vesting period for share-based award (in years)	1 year
Minimum [Member] | Restricted Stock Units [Member]
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2014	$ 22.14
Granted	23.70
Vested	22.14
Forfeited	22.17
Balance at December 31, 2015	22.52	22.14
Maximum [Member] | Restricted Stock Units [Member]
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2014	24.70
Granted	25.07
Vested	25.07
Forfeited	24.41
Balance at December 31, 2015	$ 25.07	$ 24.70
Questar Gas [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	$ 1.4	$ 1.6	1.4
Questar Gas [Member] | Restricted Stock Units [Member]
Transactions under the terms of the LTSIP - shares [Roll Forward]
Balance at December 31, 2015	97,209
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2015	$ 24.01
Share-based awards other than options, additional disclosures [Abstract]
Weighted-average remaining vesting period of share-based awards issued (in months)	10 months
Questar Gas [Member] | Minimum [Member] | Restricted Stock Units [Member]
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2015	$ 23.09
Questar Gas [Member] | Maximum [Member] | Restricted Stock Units [Member]
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2015	$ 24.41
Questar Pipeline [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	$ 2.0	$ 2.1	$ 2.0
Questar Pipeline [Member] | Restricted Stock Units [Member]
Transactions under the terms of the LTSIP - shares [Roll Forward]
Balance at December 31, 2015	126,744
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2015	$ 23.95
Share-based awards other than options, additional disclosures [Abstract]
Weighted-average remaining vesting period of share-based awards issued (in months)	11 months
Questar Pipeline [Member] | Minimum [Member] | Restricted Stock Units [Member]
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2015	$ 23.09
Questar Pipeline [Member] | Maximum [Member] | Restricted Stock Units [Member]
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2015	$ 24.41