CONSOLIDATED STATEMENTS OF INCOME (USD $)
In Millions, except Per Share data, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Mar. 31, 2015
Mar. 31, 2014
REVENUES        
Questar Gas / Revenues from unaffiliated customers $ 374.8 $ 396.3 $ 939.4 $ 963.2
Wexpro 6.3 12.3 29.6 47.1
Questar Pipeline / Revenues from unaffiliated customers 46.8 48.0 189.0 189.2
Other 0.7 0.3 3.0 0.5
Total Revenues 428.6 456.9 1,161.0 1,200.0
OPERATING EXPENSES        
Cost of sales (excluding operating expenses shown separately) 131.8 148.2 169.9 234.4
Operating and maintenance 52.3 56.9 189.6 186.7
General and administrative 29.4 30.3 121.8 113.8
Production and other taxes 13.8 17.9 62.1 60.0
Depreciation, depletion and amortization 54.3 55.3 212.7 201.6
Abandonment and impairment 0 0 2.0 80.6
Total Operating Expenses 281.6 308.6 758.1 877.1
Net gain (loss) from asset sales 0 0 1.2 (0.2)
OPERATING INCOME 147.0 148.3 404.1 322.7
Interest and other income 1.4 1.8 6.2 8.7
Income from unconsolidated affiliate 0.9 0.9 3.5 3.7
Interest expense (15.9) (15.8) (63.2) (58.2)
INCOME BEFORE INCOME TAXES 133.4 135.2 350.6 276.9
Income taxes (48.8) (50.1) (124.6) (103.5)
NET INCOME 84.6 85.1 226.0 173.4
Earnings Per Common Share        
Basic $ 0.48 $ 0.48 $ 1.29 $ 0.98
Diluted $ 0.48 $ 0.48 $ 1.28 $ 0.98
Weighted-average common shares outstanding        
Used in basic calculation 176.2 175.7 176.0 175.6
Used in diluted calculation 176.5 176.1 176.3 176.1
Dividends per common share $ 0.21 $ 0.18 $ 0.78 $ 0.72
Questar Gas [Member]        
REVENUES        
Questar Gas / Revenues from unaffiliated customers 374.8 396.3 939.4 963.2
Revenues from affiliated company 0 0 0 0.3
Total Revenues 374.8 396.3 939.4 963.5
OPERATING EXPENSES        
Cost of natural gas sold (excluding operating expenses shown separately) 233.6 254.6 583.8 614.6
Operating and maintenance 33.1 39.8 115.8 119.2
General and administrative 13.4 13.8 52.4 53.6
Depreciation and amortization 13.5 13.2 53.9 50.8
Other taxes 4.6 5.1 17.3 17.9
Total Operating Expenses 298.2 326.5 823.2 856.1
Net gain (loss) from asset sales 0 0 0.1 0
OPERATING INCOME 76.6 69.8 116.3 107.4
Interest and other income 1.1 1.3 5.7 5.2
Interest expense (7.1) (7.1) (28.2) (23.6)
INCOME BEFORE INCOME TAXES 70.6 64.0 93.8 89.0
Income taxes (26.8) (24.4) (34.4) (33.6)
NET INCOME 43.8 39.6 59.4 55.4
Questar Pipeline [Member]        
REVENUES        
Questar Pipeline / Revenues from unaffiliated customers 46.8 48.0 189.0 189.2
Revenues from affiliated companies 19.4 19.3 73.8 76.5
Total Revenues 66.2 67.3 262.8 265.7
OPERATING EXPENSES        
Operating and maintenance 10.1 9.3 40.1 38.1
General and administrative 10.5 10.3 39.1 40.9
Depreciation and amortization 14.0 13.6 54.9 55.0
Abandonment and impairment 0 0 0 80.6
Other taxes 2.3 2.3 9.1 9.1
Cost of sales (excluding operating expenses shown separately) 1.9 1.7 4.2 6.6
Total Operating Expenses 38.8 37.2 147.4 230.3
Net gain (loss) from asset sales 0 0 (0.5) 0
OPERATING INCOME 27.4 30.1 114.9 35.4
Interest and other income 0.2 0.3 1.1 1.8
Income from unconsolidated affiliate 0.9 0.9 3.5 3.7
Interest expense (6.6) (6.5) (26.2) (25.8)
INCOME BEFORE INCOME TAXES 21.9 24.8 93.3 15.1
Income taxes (8.0) (9.1) (34.5) (7.0)
NET INCOME $ 13.9 $ 15.7 $ 58.8 $ 8.1