CONSOLIDATED STATEMENTS OF INCOME - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
REVENUES
Questar Gas / Revenues from unaffiliated customers	$ 89.3	$ 100.7	$ 605.8	$ 642.0	$ 924.7	$ 979.4
Wexpro	6.4	9.4	22.8	29.5	28.9	42.6
Questar Pipeline / Revenues from unaffiliated customers	45.0	47.3	138.3	143.4	185.1	190.6
Other	1.6	0.5	3.3	1.2	4.7	1.4
Total Revenues	142.3	157.9	770.2	816.1	1,143.4	1,214.0
OPERATING EXPENSES
Cost of sales (excluding operating expenses shown separately)	(52.6)	(42.3)	70.2	81.6	174.9	210.1
Operating and maintenance	38.6	40.5	131.4	139.9	185.7	193.1
General and administrative	26.5	24.7	83.3	84.2	121.8	113.9
Production and other taxes	12.7	16.8	39.3	53.4	52.1	65.6
Depreciation, depletion and amortization	55.0	50.8	162.9	162.9	213.7	214.3
Abandonment and impairment	0.0	0.0	0.1	2.0	0.1	2.0
Total Operating Expenses	80.2	90.5	487.2	524.0	748.3	799.0
Net gain (loss) from asset sales	0.2	1.5	1.6	1.6	1.2	1.3
OPERATING INCOME (LOSS)	62.3	68.9	284.6	293.7	396.3	416.3
Interest and other income	1.3	2.0	4.2	5.0	5.8	5.7
Income from unconsolidated affiliate	0.9	0.9	2.8	2.7	3.6	3.6
Interest expense	(15.7)	(15.7)	(47.4)	(47.3)	(63.2)	(61.5)
INCOME (LOSS) BEFORE INCOME TAXES	48.8	56.1	244.2	254.1	342.5	364.1
Income taxes	(16.2)	(17.5)	(86.4)	(90.1)	(122.2)	(132.0)
NET INCOME (LOSS)	$ 32.6	$ 38.6	$ 157.8	$ 164.0	$ 220.3	$ 232.1
Earnings Per Common Share
Basic	$ 0.18	$ 0.22	$ 0.89	$ 0.93	$ 1.25	$ 1.32
Diluted	$ 0.18	$ 0.22	$ 0.89	$ 0.93	$ 1.25	$ 1.32
Weighted-average common shares outstanding
Used in basic calculation	176.2	175.8	176.3	175.8	176.2	175.7
Used in diluted calculation	176.3	176.1	176.5	176.1	176.4	176.0
Dividends per common share	$ 0.21	$ 0.19	$ 0.63	$ 0.56	$ 0.82	$ 0.74
Questar Gas [Member]
REVENUES
Questar Gas / Revenues from unaffiliated customers	$ 89.3	$ 100.7	$ 605.8	$ 642.0	$ 924.7	$ 979.4
OPERATING EXPENSES
Cost of natural gas sold (excluding operating expenses shown separately)	44.4	56.8	362.0	398.6	568.2	625.0
Operating and maintenance	22.4	22.7	80.3	88.8	114.0	123.8
General and administrative	12.9	12.3	39.1	39.0	52.9	53.0
Depreciation and amortization	13.8	13.5	40.9	40.3	54.2	53.1
Other taxes	4.7	4.8	14.4	14.7	17.5	17.3
Total Operating Expenses	98.2	110.1	536.7	581.4	806.8	872.2
Net gain (loss) from asset sales	0.0	0.0	0.0	0.1	0.0	0.1
OPERATING INCOME (LOSS)	(8.9)	(9.4)	69.1	60.7	117.9	107.3
Interest and other income	1.3	1.7	3.6	4.3	5.2	5.6
Interest expense	(7.0)	(7.1)	(21.2)	(21.2)	(28.2)	(26.8)
INCOME (LOSS) BEFORE INCOME TAXES	(14.6)	(14.8)	51.5	43.8	94.9	86.1
Income taxes	5.8	5.7	(19.3)	(16.6)	(34.7)	(32.9)
NET INCOME (LOSS)	(8.8)	(9.1)	32.2	27.2	60.2	53.2
Questar Pipeline [Member]
REVENUES
Questar Pipeline / Revenues from unaffiliated customers	45.0	47.3	138.3	143.4	185.1	190.6
Revenues from affiliated companies	17.6	17.7	54.8	54.7	73.8	76.6
Total Revenues	62.6	65.0	193.1	198.1	258.9	267.2
OPERATING EXPENSES
Operating and maintenance	8.9	10.8	27.9	29.3	37.9	39.7
General and administrative	9.0	8.9	30.0	28.7	40.2	39.0
Depreciation and amortization	13.3	13.5	41.0	40.7	54.8	54.0
Other taxes	2.1	2.4	6.5	7.1	8.5	9.0
Cost of sales (excluding operating expenses shown separately)	0.6	0.6	2.3	3.0	3.3	6.4
Total Operating Expenses	33.9	36.2	107.7	108.8	144.7	148.1
Net gain (loss) from asset sales	0.1	0.0	0.1	0.0	(0.4)	0.0
OPERATING INCOME (LOSS)	28.8	28.8	85.5	89.3	113.8	119.1
Interest and other income	0.1	0.3	0.5	0.7	1.0	1.3
Income from unconsolidated affiliate	0.9	0.9	2.8	2.7	3.6	3.6
Interest expense	(6.6)	(6.5)	(19.7)	(19.6)	(26.2)	(25.9)
INCOME (LOSS) BEFORE INCOME TAXES	23.2	23.5	69.1	73.1	92.2	98.1
Income taxes	(8.0)	(8.6)	(24.8)	(26.8)	(33.6)	(36.0)
NET INCOME (LOSS)	$ 15.2	$ 14.9	$ 44.3	$ 46.3	$ 58.6	$ 62.1