CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
OPERATING ACTIVITIES
Net income	$ 212.0	$ 207.9	$ 340.5
Discontinued operations, net of income taxes	0	0	(148.2)
Adjustments to reconcile net income to net cash provided by operating activities of continuing operations:
Depreciation, depletion and amortization	189.2	169.4	161.8
Deferred income taxes	118.6	96.2	91.4
Share-based compensation	9.9	10.1	15.4
Net (gain) from asset sales	(5.1)	(0.2)	(0.4)
(Income) from unconsolidated affiliate	(3.7)	(3.8)	(3.8)
Distributions from unconsolidated affiliate	4.5	4.4	4.0
Other operating	0.4	0.1	(1.5)
Changes in operating assets and liabilities
Accounts receivable	23.7	7.7	2.9
Inventories	2.5	(3.3)	0.1
Prepaid expenses	(2.8)	(2.1)	0
Interest rate hedge - current	7.6	(37.3)	0
Accounts payable and accrued expenses	(5.1)	11.7	(18.4)
Federal income taxes	(10.7)	37.1	(33.8)
Regulatory assets and liabilities	(24.6)	31.2	(34.8)
Other	(48.7)	(40.1)	(24.3)
Net Cash Provided By Operating Activities Of Continuing Operations	467.7	489.0	350.9
INVESTING ACTIVITIES
Property, plant and equipment	(370.7)	(367.7)	(320.3)
Equity investment in QEP	0	0	(250.0)
Cash used in disposition of assets	(3.0)	(3.5)	(2.6)
Proceeds from disposition of assets and other	8.4	0.3	0.6
Change in notes receivable	0	0	39.3
Distribution from QEP	0	0	7.3
Net Cash Used In Investing Activities Of Continuing Operations	(365.3)	(370.9)	(525.7)
FINANCING ACTIVITIES
Common stock issued	7.2	7.6	17.9
Common stock repurchased	(87.5)	(4.9)	(11.1)
Long-term debt issued, net of issuance costs	148.8	174.9	244.6
Long-term debt repaid	(91.5)	(182.0)	0
Capital lease obligation repaid	(0.8)	0	0
Change in short-term debt	44.0	(23.0)	73.0
Change in notes payable	0	0	(52.9)
Dividends paid	(117.4)	(110.1)	(94.8)
Tax benefits from share-based compensation	0	9.2	8.4
Net Cash Provided By (Used in) Financing Activities Of Continuing Operations	(97.2)	(128.3)	185.1
CASH PROVIDED BY (USED IN) CONTINUING OPERATIONS	5.2	(10.2)	10.3
Cash provided by operating activities of discontinued operations	0	0	475.5
Cash used in investing activities of discontinued operations	0	0	(598.6)
Cash provided by (used in) financing activities of discontinued operations	0	0	103.8
Effect of change in cash and cash equivalents of discontinued operations	0	0	19.3
Change in cash and cash equivalents	5.2	(10.2)	10.3
Beginning cash and cash equivalents	11.6	21.8	11.5
Ending cash and cash equivalents	16.8	11.6	21.8
Supplemental Disclosure of Cash Paid (Received) During the Year for:
Interest	58.5	54.2	55.4
Income taxes	9.1	(29.2)	44.6
Supplemental Disclosure of Noncash Investing and Financing Transaction:
Capital lease obligation incurred	40.8
Questar Gas [Member]
OPERATING ACTIVITIES
Net income	47.1	46.1	43.9
Adjustments to reconcile net income to net cash provided by operating activities of continuing operations:
Depreciation and amortization	52.1	49.2	48.0
Deferred income taxes	45.9	25.2	43.6
Share-based compensation	1.2	1.1	1.4
Changes in operating assets and liabilities
Accounts receivable	(12.7)	5.0	(4.4)
Inventories	0.7	(1.3)	3.4
Prepaid expenses	(0.1)	(0.2)	0.5
Accounts payable and accrued expenses	7.8	(1.8)	(21.6)
Federal income taxes	1.5	6.2	(8.5)
Regulatory assets and liabilities	(24.4)	37.6	(33.8)
Other	(6.4)	(20.2)	(3.2)
Net Cash Provided By Operating Activities Of Continuing Operations	112.7	146.9	69.3
INVESTING ACTIVITIES
Property, plant and equipment	(162.1)	(121.5)	(108.6)
Cash used in disposition of assets	(2.3)	(2.2)	(1.5)
Proceeds from disposition of assets and other	0.4	0.2	0.5
Affiliated-company property, plant and equipment transfers	0	0.1	0
Net Cash Used In Investing Activities Of Continuing Operations	(164.0)	(123.4)	(109.6)
FINANCING ACTIVITIES
Equity contribution	0	20.0	0
Long-term debt issued, net of issuance costs	148.8	0	0
Long-term debt repaid	(91.5)	(2.0)	0
Change in notes payable to Questar	23.4	(10.9)	66.6
Dividends paid	(33.0)	(30.3)	(28.8)
Net Cash Provided By (Used in) Financing Activities Of Continuing Operations	47.7	(23.2)	37.8
Change in cash and cash equivalents	(3.6)	0.3	(2.5)
Beginning cash and cash equivalents	5.0	4.7	7.2
Ending cash and cash equivalents	1.4	5.0	4.7
Supplemental Disclosure of Cash Paid (Received) During the Year for:
Interest	24.7	25.1	25.5
Income taxes	(20.4)	(3.3)	(9.7)
Questar Pipeline [Member]
OPERATING ACTIVITIES
Net income	64.7	67.9	67.4
Adjustments to reconcile net income to net cash provided by operating activities of continuing operations:
Depreciation and amortization	56.8	53.9	49.9
Deferred income taxes	41.1	34.5	17.2
Share-based compensation	2.0	1.8	1.3
Net (gain) from asset sales	(2.7)	(0.3)	(0.8)
(Income) from unconsolidated affiliate	(3.7)	(3.8)	(3.8)
Distributions from unconsolidated affiliate	4.5	4.4	4.0
Other operating	0.4	0.1	(1.5)
Changes in operating assets and liabilities
Accounts receivable	(2.1)	(0.8)	(2.0)
Inventories	0.2	(0.2)	(1.2)
Prepaid expenses	(3.3)	0	(0.3)
Interest rate hedge - current	0	(37.3)	0
Accounts payable and accrued expenses	(3.4)	0.8	8.5
Federal income taxes	0	1.1	0.6
Regulatory assets and liabilities	(0.9)	(5.0)	1.0
Net Cash Provided By Operating Activities Of Continuing Operations	153.6	117.1	140.3
INVESTING ACTIVITIES
Property, plant and equipment	(60.6)	(104.5)	(121.0)
Cash used in disposition of assets	(0.7)	(1.3)	(1.1)
Proceeds from disposition of assets and other	4.3	0.1	0.1
Affiliated-company property, plant and equipment transfers	0	(0.1)	0
Net Cash Used In Investing Activities Of Continuing Operations	(57.0)	(105.8)	(122.0)
FINANCING ACTIVITIES
Long-term debt issued, net of issuance costs	0	176.2	0
Long-term debt repaid	0	(180.0)	0
Change in notes receivable from Questar	(29.9)	21.5	12.4
Change in notes payable to Questar	0	0	(0.2)
Dividends paid	(63.0)	(30.9)	(29.0)
Net Cash Provided By (Used in) Financing Activities Of Continuing Operations	(92.9)	(13.2)	(16.8)
Change in cash and cash equivalents	3.7	(1.9)	1.5
Beginning cash and cash equivalents	3.4	5.3	3.8
Ending cash and cash equivalents	7.1	3.4	5.3
Supplemental Disclosure of Cash Paid (Received) During the Year for:
Interest	25.2	24.7	29.0
Income taxes	$ (3.8)	$ 2.9	$ 19.2