CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
OPERATING ACTIVITIES
Net income	$ 112.3	$ 114.4
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	99.5	93.6
Deferred income taxes	53.7	60.6
Share-based compensation	8.3	5.2
Net (gain) from asset sales	(0.1)	(2.6)
(Income) from unconsolidated affiliate	(1.9)	(1.8)
Distributions from unconsolidated affiliate and other	3.0	2.2
Changes in operating assets and liabilities	62.8	33.4
Net Cash Provided By Operating Activities	337.6	305.0
INVESTING ACTIVITIES
Property, plant and equipment	(192.9)	(190.8)
Cash used in disposition of assets	(2.2)	(1.7)
Proceeds from disposition of assets	0.5	0.4
Net Cash Used In Investing Activities	(194.6)	(192.1)
FINANCING ACTIVITIES
Common stock issued	2.2	3.8
Common stock repurchased	(3.5)	(56.8)
Change in short-term debt	(57.0)	(29.0)
Long-term debt and capital lease obligation repaid	(40.3)	(0.4)
Long-term debt issuance costs	(1.7)	0
Checks outstanding in excess of cash balances	0.5	11.8
Dividends paid	(61.4)	(57.8)
Tax benefits from share-based compensation	1.4	3.9
Net Cash Provided By (Used In) Financing Activities	(159.8)	(124.5)
Change in cash and cash equivalents	(16.8)	(11.6)
Beginning cash and cash equivalents	16.8	11.6
Ending cash and cash equivalents	0	0
Supplemental Disclosure of Noncash Investing and Financing Transaction:
Capital lease obligation incurred		40.8
Questar Gas [Member]
OPERATING ACTIVITIES
Net income	35.8	32.6
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	26.9	26.1
Deferred income taxes	17.8	0.8
Share-based compensation	1.0	0.6
Changes in operating assets and liabilities	65.1	73.9
Net Cash Provided By Operating Activities	146.6	134.0
INVESTING ACTIVITIES
Property, plant and equipment	(74.1)	(85.9)
Cash used in disposition of assets	(1.8)	(1.6)
Proceeds from disposition of assets	0.2	0.2
Affiliated-company property, plant and equipment transfers	10.8	0
Net Cash Used In Investing Activities	(64.9)	(87.3)
FINANCING ACTIVITIES
Long-term debt and capital lease obligation repaid	(40.0)	0
Change in notes payable to Questar	(28.8)	(40.6)
Checks outstanding in excess of cash balances	3.4	5.4
Dividends paid	(17.7)	(16.5)
Net Cash Provided By (Used In) Financing Activities	(83.1)	(51.7)
Change in cash and cash equivalents	(1.4)	(5.0)
Beginning cash and cash equivalents	1.4	5.0
Ending cash and cash equivalents	0	0
Questar Pipeline [Member]
OPERATING ACTIVITIES
Net income	30.3	32.7
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	29.6	28.3
Deferred income taxes	5.6	3.5
Share-based compensation	1.6	1.0
Net (gain) from asset sales	0	(0.1)
(Income) from unconsolidated affiliate	(1.9)	(1.8)
Distributions from unconsolidated affiliate and other	2.8	2.2
Changes in operating assets and liabilities	1.6	(2.2)
Net Cash Provided By Operating Activities	69.6	63.6
INVESTING ACTIVITIES
Property, plant and equipment	(32.3)	(25.3)
Cash used in disposition of assets	(0.4)	(0.1)
Proceeds from disposition of assets	0.1	0.2
Affiliated-company property, plant and equipment transfers	(10.7)	0
Net Cash Used In Investing Activities	(43.3)	(25.2)
FINANCING ACTIVITIES
Change in notes receivable from Questar	4.3	(10.0)
Dividends paid	(32.0)	(31.5)
Net Cash Provided By (Used In) Financing Activities	(27.7)	(41.5)
Change in cash and cash equivalents	(1.4)	(3.1)
Beginning cash and cash equivalents	7.1	3.4
Ending cash and cash equivalents	$ 5.7	$ 0.3