CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
OPERATING ACTIVITIES
Net income	$ 72.9	$ 75.2
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	50.2	46.3
Deferred income taxes	25.4	39.6
Share-based compensation	3.1	2.7
Net (gain) loss from asset sales	0	(2.2)
(Income) from unconsolidated affiliate	(0.9)	(0.9)
Distributions from unconsolidated affiliate and other	1.5	1.3
Changes in operating assets and liabilities	53.8	58.7
Net Cash Provided By Operating Activities	206.0	220.7
INVESTING ACTIVITIES
INVESTING ACTIVITIES	(84.0)	(104.2)
Cash used in disposition of assets	(0.5)	(0.7)
Proceeds from disposition of assets	0.1	0
Net Cash Used In Investing Activities	(84.4)	(104.9)
FINANCING ACTIVITIES
Common stock issued	1.8	2.8
Common stock repurchased	(3.5)	(6.6)
Change in short-term debt	(61.0)	(90.0)
Long-term debt and capital lease obligation repaid	(40.1)	0
Dividends paid	(29.8)	(29.0)
Tax benefits from share-based compensation	1.4	3.5
Net Cash Provided By (Used In) Financing Activities	(131.2)	(119.3)
Change in cash and cash equivalents	(9.6)	(3.5)
Beginning cash and cash equivalents	16.8	11.6
Ending cash and cash equivalents	7.2	8.1
Supplemental Disclosure of Noncash Investing and Financing Transaction:
Capital lease obligation incurred		40.8
Questar Gas [Member]
OPERATING ACTIVITIES
Net income	37.0	34.9
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	13.4	13.0
Deferred income taxes	10.7	(2.0)
Share-based compensation	0.4	0.3
Changes in operating assets and liabilities	44.0	83.3
Net Cash Provided By Operating Activities	105.5	129.5
INVESTING ACTIVITIES
INVESTING ACTIVITIES	(31.7)	(44.8)
Cash used in disposition of assets	(0.4)	(0.6)
Affiliated-company property, plant and equipment transfers	10.7	0
Net Cash Used In Investing Activities	(21.4)	(45.4)
FINANCING ACTIVITIES
Long-term debt and capital lease obligation repaid	(40.0)	0
Change in notes payable to Questar	(33.8)	(76.3)
Dividends paid	(8.9)	(8.3)
Net Cash Provided By (Used In) Financing Activities	(82.7)	(84.6)
Change in cash and cash equivalents	1.4	(0.5)
Beginning cash and cash equivalents	1.4	5.0
Ending cash and cash equivalents	2.8	4.5
Questar Pipeline [Member]
OPERATING ACTIVITIES
Net income	15.8	16.6
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	14.7	14.0
Deferred income taxes	3.2	9.1
Share-based compensation	0.7	0.6
(Income) from unconsolidated affiliate	(0.9)	(0.9)
Distributions from unconsolidated affiliate and other	1.5	1.3
Changes in operating assets and liabilities	(1.4)	(5.9)
Net Cash Provided By Operating Activities	33.6	34.8
INVESTING ACTIVITIES
INVESTING ACTIVITIES	(14.1)	(12.1)
Cash used in disposition of assets	(0.1)	(0.1)
Affiliated-company property, plant and equipment transfers	(10.7)	0
Net Cash Used In Investing Activities	(24.9)	(12.2)
FINANCING ACTIVITIES
Change in notes receivable from Questar	5.6	(5.7)
Dividends paid	(16.0)	(15.7)
Net Cash Provided By (Used In) Financing Activities	(10.4)	(21.4)
Change in cash and cash equivalents	(1.7)	1.2
Beginning cash and cash equivalents	7.1	3.4
Ending cash and cash equivalents	$ 5.4	$ 4.6