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Share-Based Compensation Performance shares (Details)
$ / shares in Units, $ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2016
$ / shares
shares
Dec. 31, 2015
USD ($)
$ / shares
shares
Dec. 31, 2014
USD ($)
$ / shares
shares
Dec. 31, 2013
USD ($)
$ / shares
Performance Shares [Member]        
Major assumptions used in the calculation of fair value of performance shares granted [Abstract]        
Weighted-average grant date fair value, performance shares granted (in dollars per share)   $ 21.03    
Weighted-average grant date fair value, performance shares granted (in dollars per share)   $ 20.82 $ 31.07 $ 39.62
Risk-free interest rate (in hundredths)   0.95% 0.67% 0.40%
Expected price volatility (in hundredths)   16.10% 18.60% 19.00%
Expected dividend yield (in hundredths)   3.11% 3.05% 2.88%
Expected life (in years and months)   2 years 11 months 2 years 11 months 2 years 11 months
Transactions under the terms of the LTSIP - shares [Roll Forward]        
Balance at December 31, 2014 | shares 343,748 [1],[2] 391,249    
Granted | shares   160,302    
Distributed | shares [3]   (134,778)    
Forfeited | shares   (73,025)    
Balance at December 31, 2015 | shares   343,748 [1],[2] 391,249  
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]        
Balance at December 31, 2014 $ 30.37 [1],[2] $ 31.78    
Granted   20.82 $ 31.07 $ 39.62
Distributed [3]   25.42    
Forfeited   26.11    
Performance shares, additional disclosures [Abstract]        
Balance at December 31, 2015   $ 30.37 [1],[2] $ 31.78  
Performance share payout adjustment multiplier   1.00    
Performance period that the target number of performance shares for each exectutive officer is subject to adjustment based on the company's performance (in years)   3 years    
Equity- and liability-based performance share compensation expense | $   $ 2.8 $ 3.8 $ 2.0
Weighted-average remaining vesting period of share-based awards issued (in months)   19 months    
Performance share cash distributed | $   $ 1.7    
Performance Shares [Member] | Vested Undistributed [Member]        
Performance shares, additional disclosures [Abstract]        
Performance share payout adjustment multiplier 0.32      
Number of undistributed performance shares at end of period (in shares) | shares   115,074    
The date the share-based award will be distributed first quarter of 2016      
Performance Shares [Member] | Minimum [Member]        
Major assumptions used in the calculation of fair value of performance shares granted [Abstract]        
Weighted-average grant date fair value, performance shares granted (in dollars per share)   $ 19.03    
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]        
Balance at December 31, 2014 $ 19.03 [1],[2] 25.42    
Granted   19.03    
Distributed [3]   25.42    
Forfeited   21.03    
Performance shares, additional disclosures [Abstract]        
Balance at December 31, 2015   $ 19.03 [1],[2] $ 25.42  
Performance share payout adjustment multiplier   0.00    
Performance Shares [Member] | Maximum [Member]        
Major assumptions used in the calculation of fair value of performance shares granted [Abstract]        
Weighted-average grant date fair value, performance shares granted (in dollars per share)   $ 21.03    
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]        
Balance at December 31, 2014 $ 39.62 [1],[2] 39.62    
Granted   21.03    
Distributed [3]   25.42    
Forfeited   39.62    
Performance shares, additional disclosures [Abstract]        
Balance at December 31, 2015   $ 39.62 [1],[2] $ 39.62  
Performance share payout adjustment multiplier   3.00    
Performance Shares - Cash Settled [Member]        
Performance shares, additional disclosures [Abstract]        
Number of undistributed performance shares at end of period (in shares) | shares   57,537    
Performance Shares - Equity Settled [Member]        
Performance shares, additional disclosures [Abstract]        
Number of undistributed performance shares at end of period (in shares) | shares   286,211    
Questar Gas [Member] | Performance Shares [Member]        
Transactions under the terms of the LTSIP - shares [Roll Forward]        
Balance at December 31, 2014 | shares 33,578      
Balance at December 31, 2015 | shares   33,578    
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]        
Balance at December 31, 2014 $ 30.23      
Performance shares, additional disclosures [Abstract]        
Balance at December 31, 2015   $ 30.23    
Weighted-average remaining vesting period of share-based awards issued (in months)   18 months    
Questar Gas [Member] | Performance Shares [Member] | Minimum [Member]        
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]        
Balance at December 31, 2014 21.03      
Performance shares, additional disclosures [Abstract]        
Balance at December 31, 2015   $ 21.03    
Questar Gas [Member] | Performance Shares [Member] | Maximum [Member]        
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]        
Balance at December 31, 2014 $ 39.62      
Performance shares, additional disclosures [Abstract]        
Balance at December 31, 2015   $ 39.62    
Questar Pipeline [Member] | Performance Shares [Member]        
Transactions under the terms of the LTSIP - shares [Roll Forward]        
Balance at December 31, 2014 | shares 47,454      
Balance at December 31, 2015 | shares   47,454    
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]        
Balance at December 31, 2014 $ 28.43      
Performance shares, additional disclosures [Abstract]        
Balance at December 31, 2015   $ 28.43    
Weighted-average remaining vesting period of share-based awards issued (in months)   23 months    
Questar Pipeline [Member] | Performance Shares [Member] | Minimum [Member]        
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]        
Balance at December 31, 2014 19.03      
Performance shares, additional disclosures [Abstract]        
Balance at December 31, 2015   $ 19.03    
Questar Pipeline [Member] | Performance Shares [Member] | Maximum [Member]        
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]        
Balance at December 31, 2014 $ 39.62      
Performance shares, additional disclosures [Abstract]        
Balance at December 31, 2015   $ 39.62    
[1] The total undistributed balance consists of 286,211 of share-settled target shares and 57,537 of cash-settled target shares.
[2] The total undistributed balance includes 115,074 vested target shares that are expected to be distributed in the first quarter of 2016 using a payout adjustment multiplier of 0.32. The multiplier reflects Questar's final total shareholder return ranking relative to the specified peer companies for the performance period from January 1, 2013 through December 31, 2015. The distribution will be half in shares, half in cash. These target shares are included in the amounts described in note (2), immediately above.
[3] Actual shares and cash distributed were determined by multiplying the target shares by 1.00 to reflect Questar's final total shareholder return ranking relative to the specified peer companies for the performance period from January 1, 2012 through December 31, 2014. The total amount paid for the cash half of the distribution was $1.7 million.