Questar Gas 10-K 20 Feb 15 2014 FY Annual report Financial data

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
	Dec. 31, 2014	Jan. 31, 2015	Jun. 30, 2014
Entity Information [Line Items]
Entity Registrant Name	QUESTAR CORP
Entity Central Index Key	751652
Current Fiscal Year End Date	-19
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$4,300,000,000
Entity Common Stock, Shares Outstanding		175,494,622
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Document Type	10-K
Amendment Flag	FALSE
Document Period End Date	31-Dec-14
Questar Gas [Member]
Entity Information [Line Items]
Entity Registrant Name	QUESTAR GAS CO
Entity Central Index Key	68589
Current Fiscal Year End Date	-19
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Public Float			0
Entity Common Stock, Shares Outstanding		9,189,626
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Document Type	10-K
Amendment Flag	FALSE
Document Period End Date	31-Dec-14
Questar Pipeline [Member]
Entity Information [Line Items]
Entity Registrant Name	QUESTAR PIPELINE CO
Entity Central Index Key	764044
Current Fiscal Year End Date	-19
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Public Float			$0
Entity Common Stock, Shares Outstanding		6,550,843
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Document Type	10-K
Amendment Flag	FALSE
Document Period End Date	31-Dec-14

CONSOLIDATED_STATEMENTS_OF_INC

CONSOLIDATED STATEMENTS OF INCOME (USD $)	12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
REVENUES
Questar Gas / Revenues from unaffiliated customers	$960.90	$985.20	$859.70
Wexpro	35.6	45.1	36.1
Questar Pipeline / Revenues from unaffiliated customers	190.2	189.5	203.1
Other	2.6	0.2	0
Total Revenues	1,189.30	1,220	1,098.90
OPERATING EXPENSES
Cost of sales (excluding operating expenses shown separately)	186.3	285.9	192.3
Cost of natural gas sold
Operating and maintenance	189	174.3	180.8
General and administrative	127.9	121	120.8
Retirement incentive	0	0	4.9
Production and other taxes	66.2	57.4	47.9
Depreciation, depletion and amortization	213.7	194.8	181.6
Abandonment and impairment	2	80.6	0
Total Operating Expenses	785.1	914	728.3
Net gain (loss) from asset sales	1.2	-0.2	5.1
OPERATING INCOME	405.4	305.8	375.7
Interest and other income	6.6	9.9	7
Income from unconsolidated affiliate	3.5	3.7	3.7
Interest expense	-63.1	-56.9	-57.9
INCOME BEFORE INCOME TAXES	352.4	262.5	328.5
Income taxes	-125.9	-101.3	-116.5
NET INCOME	226.5	161.2	212
Earnings Per Common Share
Basic	$1.29	$0.92	$1.20
Diluted	$1.29	$0.92	$1.19
Weighted-average common shares outstanding
Used in basic calculation	175.8	175.4	176.5
Used in diluted calculation	176.1	176	177.5
Questar Gas [Member]
REVENUES
Questar Gas / Revenues from unaffiliated customers	960.9	985.2	859.7
Revenues from affiliated company	0	0.6	2.5
Total Revenues	960.9	985.8	862.2
Cost of natural gas sold
Cost of natural gas sold - unaffiliated parties	181.4	279.7	185.6
Cost of natural gas sold - affiliated companies	423.4	370.9	347.7
Total cost of natural gas sold (excluding operating expenses shown separately) / Total Questar Gas cost of natural gas sold	604.8	650.6	533.3
Operating and maintenance	122.5	113.1	119
General and administrative	52.8	52.5	51.2
Retirement incentive	0	0	2.4
Depreciation and amortization	53.6	49.7	47.2
Other taxes	17.8	18	16.2
Total Operating Expenses	851.5	883.9	769.3
Net gain (loss) from asset sales	0.1	0	0
OPERATING INCOME	109.5	101.9	92.9
Interest and other income	5.9	5.1	5.5
Interest expense	-28.2	-22.3	-24.1
INCOME BEFORE INCOME TAXES	87.2	84.7	74.3
Income taxes	-32	-31.9	-27.2
NET INCOME	55.2	52.8	47.1
Questar Pipeline [Member]
REVENUES
Questar Pipeline / Revenues from unaffiliated customers	190.2	189.5	203.1
Revenues from affiliated companies	73.7	76.7	74.4
Total Revenues	263.9	266.2	277.5
Cost of natural gas sold
Operating and maintenance	34.1	32.5	35.2
General and administrative	44.1	46.7	50
Retirement incentive	0	0	0.9
Depreciation and amortization	54.5	55.5	54.3
Abandonment and impairment	0	80.6	0
Other taxes	9.1	9.3	9.1
Cost of sales (excluding operating expenses shown separately)	4	6.1	6.7
Total Operating Expenses	145.8	230.7	156.2
Net gain (loss) from asset sales	-0.5	0	2.7
OPERATING INCOME	117.6	35.5	124
Interest and other income	1.2	1.8	0.6
Income from unconsolidated affiliate	3.5	3.7	3.7
Interest expense	-26.1	-25.8	-26.3
INCOME BEFORE INCOME TAXES	96.2	15.2	102
Income taxes	-35.6	-7	-37.3
NET INCOME	$60.60	$8.20	$64.70

CONSOLIDATED_STATEMENTS_OF_COM

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net income	$226.50	$161.20	$212
Pension and other postretirement benefits
Change in unrecognized actuarial loss	-97.5	167.7	-35.7
Change in unrecognized prior service cost	0.6	1.1	2.7
Interest rate cash flow hedge amortization	0.5	0.5	0.4
Change in fair value of long-term investment	0	-0.1	0.1
Income taxes	36.9	-64.7	12.3
Net other comprehensive income (loss)	-59.5	104.5	-20.2
COMPREHENSIVE INCOME	167	265.7	191.8
Questar Pipeline [Member]
Net income	60.6	8.2	64.7
Pension and other postretirement benefits
Interest rate cash flow hedge amortization	0.5	0.5	0.4
Income taxes	-0.1	-0.2	-0.2
Net other comprehensive income (loss)	0.4	0.3	0.2
COMPREHENSIVE INCOME	$61	$8.50	$64.90

CONSOLIDATED_BALANCE_SHEETS

CONSOLIDATED BALANCE SHEETS (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Current Assets
Cash and cash equivalents	$32	$16
Accounts receivable, net	89.6	119.3
Unbilled gas accounts receivable	93.7	93.4
Federal income taxes receivable	28.5	4.5
Inventories, at lower of average cost or market:
Gas stored underground	43.7	40
Materials and supplies	30.4	23.7
Current regulatory assets	79.6	35.8
Prepaid expenses and other	11.2	9.5
Deferred income taxes - current	5.8	9.7
Total Current Assets	414.5	351.9
Property, Plant and Equipment
Distribution property, plant and equipment	2,352.30	2,203
Cost-of-service gas and oil property, plant and equipment, successful efforts method	1,688.40	1,626.40
Transportation property, plant and equipment	1,827.70	1,774.30
Other property, plant and equipment	93.1	71
Total Property, Plant and Equipment	5,961.50	5,674.70
Distribution accumulated depreciation and amortization	-780.3	-745.2
Cost-of-service gas and oil accumulated depreciation, depletion and amortization	-757.3	-689.8
Transportation accumulated depreciation and amortization	-673.9	-627.2
Other accumulated depreciation and amortization	-14.5	-9.5
Total accumulated depreciation, depletion and amortization	-2,226	-2,071.70
Net Property, Plant and Equipment	3,735.50	3,603
Investment in unconsolidated affiliate	24.7	25.6
Other Assets
Goodwill	9.8	9.8
Noncurrent regulatory assets	25	20.5
Other noncurrent assets	40.2	43.5
Total Other Assets	75	73.8
TOTAL ASSETS	4,249.70	4,054.30
Current Liabilities
Short-term debt	347	276
Accounts payable	116	158
Accrued expenses and other	42.3	42.1
Production and other taxes	35.5	34.7
Customer advances	29.4	19.8
Current regulatory liabilities	13.4	14.1
Interest payable	11.7	10.2
Current portion of long-term debt	25.1	0
Current portion of capital lease obligation	1	0.9
Total Current Liabilities	621.4	555.8
Long-term debt, less current portion	1,220.10	1,247.10
Capital lease obligation, less current portion	37.4	38.4
Deferred income taxes	715.6	707.2
Asset retirement obligations	69.3	67.7
Defined benefit pension plan	141.9	70.7
Other postretirement benefits	43.8	42.5
Noncurrent regulatory liabilities	69.9	61.2
Customer contributions in aid of construction	29.2	29.1
Other noncurrent liabilities	54.9	35.8
Commitments and contingencies - Note 9
COMMON SHAREHOLDERS' EQUITY
Common stock	476.8	464.5
Retained earnings	970.7	876.1
Accumulated other comprehensive loss	-201.3	-141.8
Total Common Shareholders' Equity	1,246.20	1,198.80
TOTAL LIABILITIES AND COMMON SHAREHOLDERS' EQUITY	4,249.70	4,054.30
Questar Gas [Member]
Current Assets
Cash and cash equivalents	19.8	8.8
Accounts receivable, net	66.4	86.1
Unbilled gas accounts receivable	93.7	93.4
Accounts receivable from affiliates	45.2	30.4
Federal income taxes receivable	0	2.8
Inventories, at lower of average cost or market:
Gas stored underground	40.3	39.2
Materials and supplies	19.2	12.1
Current regulatory assets	78.3	30.2
Prepaid expenses and other	3.5	3
Deferred income taxes - current	0	2.9
Total Current Assets	366.4	308.9
Property, Plant and Equipment
Distribution property, plant and equipment	1,991.60	1,839.80
Other property, plant and equipment	306.3	289.8
Construction work in progress	54.4	73.4
Total Property, Plant and Equipment	2,352.30	2,203
Total accumulated depreciation, depletion and amortization	-780.3	-745.2
Net Property, Plant and Equipment	1,572	1,457.80
Other Assets
Goodwill	5.6	5.6
Noncurrent regulatory assets	21.3	16.1
Other noncurrent assets	3.7	4
Total Other Assets	30.6	25.7
TOTAL ASSETS	1,969	1,792.40
Current Liabilities
Notes payable to Questar	119.3	17.7
Accounts payable	76	110.8
Accounts payable to affiliates	78.7	67.8
Accrued expenses and other	22.7	23.2
Dividends payable to Questar	9	0
Customer advances	29.4	19.8
Federal income taxes payable	6.4	0
Current regulatory liabilities	12.5	11.1
Interest payable	6.8	5.2
Deferred income taxes - current	6.3	0
Total Current Liabilities	367.1	255.6
Long-term debt, less current portion	534.5	534.5
Deferred income taxes	377.5	340.7
Asset retirement obligations	0.6	0.6
Noncurrent regulatory liabilities	60.9	53
Customer contributions in aid of construction	29.2	29.1
Other noncurrent liabilities	2.8	3.2
Commitments and contingencies - Note 9
COMMON SHAREHOLDERS' EQUITY
Common stock	23	23
Additional paid-in capital	265.4	263.9
Retained earnings	308.6	289.4
Total Common Shareholders' Equity	597	576.3
TOTAL LIABILITIES AND COMMON SHAREHOLDERS' EQUITY	1,969	1,792.40
Questar Pipeline [Member]
Current Assets
Cash and cash equivalents	7.4	2.7
Notes receivable from Questar	40.1	29.4
Accounts receivable, net	17.2	18.5
Accounts receivable from affiliates	39.2	36.6
Federal income taxes receivable	0.7	0
Inventories, at lower of average cost or market:
Materials and supplies	10.3	8.1
Current regulatory assets	1.3	5.6
Prepaid expenses and other	4.3	4.4
Deferred income taxes - current	1.9	1.8
Total Current Assets	122.4	107.1
Property, Plant and Equipment
Transportation property, plant and equipment	1,371.60	1,331
Other property, plant and equipment	82.7	82.1
Storage	300.3	296.2
Processing	24.9	24.1
Construction work in progress	48.2	40.9
Total Property, Plant and Equipment	1,827.70	1,774.30
Total accumulated depreciation, depletion and amortization	-673.9	-627.2
Net Property, Plant and Equipment	1,153.80	1,147.10
Investment in unconsolidated affiliate	24.7	25.6
Other Assets
Goodwill	4.2	4.2
Noncurrent regulatory assets	3.7	4.4
Noncurrent regulatory and other assets	6.6	7.7
Total Other Assets	10.8	11.9
TOTAL ASSETS	1,311.70	1,291.70
Current Liabilities
Accounts payable	9.7	7.5
Accounts payable to affiliates	5.8	9.4
Accrued expenses and other	9.9	13.1
Dividends payable to Questar	16	0
Federal income taxes payable	0	2.9
Current regulatory liabilities	0.9	3
Interest payable	2.1	2.1
Current portion of long-term debt	25.1	0
Total Current Liabilities	69.5	38
Long-term debt, less current portion	433.7	458.9
Deferred income taxes	241.4	228.6
Asset retirement obligations	2.3	2.2
Noncurrent regulatory liabilities	9	8.2
Noncurrent regulatory and other liabilities	15.9	13.9
Commitments and contingencies - Note 9
COMMON SHAREHOLDERS' EQUITY
Common stock	6.6	6.6
Additional paid-in capital	351.4	349.5
Retained earnings	215.7	219.1
Accumulated other comprehensive loss	-22.5	-22.9
Total Common Shareholders' Equity	551.2	552.3
TOTAL LIABILITIES AND COMMON SHAREHOLDERS' EQUITY	$1,311.70	$1,291.70

CONSOLIDATED_BALANCE_SHEETS_Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Common Stock, No Par Value	$0
Common Stock, Shares Authorized	500,000,000	500,000,000
Common Stock, Shares Outstanding	175,400,000	175,100,000
Questar Gas [Member]
Common Stock, Par or Stated Value Per Share	$2.50
Common Stock, Shares Authorized	50,000,000	50,000,000
Common Stock, Shares Issued	9,200,000	9,200,000
Common Stock, Shares Outstanding	9,200,000	9,200,000
Questar Pipeline [Member]
Common Stock, Par or Stated Value Per Share	$1
Common Stock, Shares Authorized	25,000,000	25,000,000
Common Stock, Shares Issued	6,600,000	6,600,000
Common Stock, Shares Outstanding	6,600,000	6,600,000

CONSOLIDATED_STATEMENTS_OF_COM1

CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS' EQUITY (USD $)	Total	Common Stock Including Additional Paid in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]	Questar Gas [Member]	Questar Gas [Member]	Questar Gas [Member]	Questar Gas [Member]	Questar Pipeline [Member]	Questar Pipeline [Member]	Questar Pipeline [Member]	Questar Pipeline [Member]	Questar Pipeline [Member]
In Millions						Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]		Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]
Beginning balances at Dec. 31, 2011	$1,033.50	$514.70	$744.90	($226.10)	$452.40	$23	$171.40	$258	$602.10	$6.60	$345.70	$273.20	($23.40)
Beginning balances (in shares) at Dec. 31, 2011		177.9
Common stock issued	7.2	7.2	0	0
Common stock issued (in shares)		1.5
Common stock repurchased	-88.5	-88.5	0	0
Common stock repurchased (in shares)		-4.4
Dividends paid	-117.4	0	-117.4	0	-33	0	0	-33	-63	0	0	-63	0
Share-based compensation	9	9	0	0	1.1	0	1.1	0	1.8	0	1.8	0	0
Net income	212	0	212	0	47.1	0	0	47.1	64.7	0	0	64.7	0
Net other comprehensive income (loss)	-20.2	0	0	-20.2					0.2	0	0	0	0.2
Ending balances at Dec. 31, 2012	1,035.60	442.4	839.5	-246.3	467.6	23	172.5	272.1	605.8	6.6	347.5	274.9	-23.2
Ending balances (in shares) at Dec. 31, 2012		175
Common stock issued	2.3	2.3	0	0
Common stock issued (in shares)		0.3
Common stock repurchased	-4.2	-4.2	0	0
Common stock repurchased (in shares)		-0.2
Dividends paid	-124.6	0	-124.6	0	-35.5	0	0	-35.5	-64	0	0	-64	0
Share-based compensation	10	10	0	0	1.4	0	1.4	0	2	0	2	0	0
Tax benefits from share-based compensation	14	14	0	0
Equity contribution from Questar					90	0	90	0
Net income	161.2	0	161.2	0	52.8	0	0	52.8	8.2	0	0	8.2	0
Net other comprehensive income (loss)	104.5	0	0	104.5					0.3	0	0	0	0.3
Ending balances at Dec. 31, 2013	1,198.80	464.5	876.1	-141.8	576.3	23	263.9	289.4	552.3	6.6	349.5	219.1	-22.9
Ending balances (in shares) at Dec. 31, 2013		175.1
Common stock issued	2.9	2.9	0	0
Common stock issued (in shares)		0.5
Common stock repurchased	-4.1	-4.1	0	0
Common stock repurchased (in shares)		-0.2
Dividends paid	-131.9	0	-131.9	0	-36	0	0	-36	-64	0	0	-64	0
Share-based compensation	11.3	11.3	0	0	1.5	0	1.5	0	1.9	0	1.9	0	0
Tax benefits from share-based compensation	2.2	2.2	0	0
Net income	226.5	0	226.5	0	55.2	0	0	55.2	60.6	0	0	60.6	0
Net other comprehensive income (loss)	-59.5	0	0	-59.5					0.4	0	0	0	0.4
Ending balances at Dec. 31, 2014	$1,246.20	$476.80	$970.70	($201.30)	$597	$23	$265.40	$308.60	$551.20	$6.60	$351.40	$215.70	($22.50)
Ending balances (in shares) at Dec. 31, 2014		175.4

CONSOLIDATED_STATEMENTS_OF_COM2

CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS' EQUITY (Parenthetical) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Stockholders' Equity [Abstract]
Dividends paid per common share	$0.75	$0.71	$0.67

CONSOLIDATED_STATEMENTS_OF_CAS

CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
OPERATING ACTIVITIES
Net income	$226.50	$161.20	$212
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	234.4	201.8	189.2
Deferred income taxes	49.2	42.4	118.6
Abandonment and impairment	2	80.6	0
Share-based compensation	12.4	10.2	9.9
Net (gain) loss from asset sales	-1.2	0.2	-5.1
(Income) from unconsolidated affiliate	-3.5	-3.7	-3.7
Distributions from unconsolidated affiliate	4.4	4.6	4.5
Other operating	0.9	0.9	0.4
Changes in operating assets and liabilities
Accounts receivable, including unbilled gas sales	29.5	-41.5	23.7
Inventories	-10.4	-0.2	2.5
Prepaid expenses and other	-1.7	3.6	-2.8
Interest rate hedges - current	0	0	7.6
Accounts payable and accrued expenses	-19.6	33.7	-5.1
Federal income taxes	-24	6.2	-10.7
Current regulatory assets and liabilities	-44.5	19.2	-24.6
Noncurrent regulatory and other assets and liabilities	-10.8	-17.1	-48.7
NET CASH PROVIDED BY OPERATING ACTIVITIES	443.6	502.1	467.7
INVESTING ACTIVITIES
Property, plant and equipment	-319.1	-399.4	-370.7
Wexpro acquisition of producing properties	-52.4	-104.3	0
Cash used in disposition of assets	-4.8	-4.9	-3
Proceeds from disposition of assets	9.5	0.6	8.4
NET CASH USED IN INVESTING ACTIVITIES	-366.8	-508	-365.3
FINANCING ACTIVITIES
Common stock issued	2.9	2.3	7.2
Common stock repurchased	-4.1	-4.2	-87.5
Long-term debt issued, net of issuance costs	0	147.3	148.8
Long-term debt repaid	0	-42	-91.5
Capital lease obligation repaid	-0.9	-0.7	-0.8
Change in short-term debt	71	13	44
Dividends paid	-131.9	-124.6	-117.4
Tax benefits from share-based compensation	2.2	14	0
NET CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES	-60.8	5.1	-97.2
Change in cash and cash equivalents	16	-0.8	5.2
Beginning cash and cash equivalents	16	16.8	11.6
Ending cash and cash equivalents	32	16	16.8
Supplemental Disclosure of Cash Paid (Received) During the Year for:
Interest	61.3	55.8	58.5
Income taxes	97.4	33.6	9.1
Supplemental Disclosure of Noncash Investing and Financing Transaction:
Capital lease obligation incurred			40.8
Questar Gas [Member]
OPERATING ACTIVITIES
Net income	55.2	52.8	47.1
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	58.8	54.7	52.1
Deferred income taxes	46	37.7	45.9
Share-based compensation	1.6	1.4	1.2
Net (gain) loss from asset sales	-0.1	0	0
Changes in operating assets and liabilities
Accounts receivable, including unbilled gas sales	4.6	-43.7	-12.7
Inventories	-8.2	0.5	0.7
Prepaid expenses and other	-0.5	0.2	-0.1
Accounts payable and accrued expenses	-0.2	29.5	7.8
Federal income taxes	9.2	-2.8	1.5
Current regulatory assets and liabilities	-46.7	17.6	-24.4
Noncurrent regulatory and other assets and liabilities	-6	5.4	-6.4
NET CASH PROVIDED BY OPERATING ACTIVITIES	113.7	153.3	112.7
INVESTING ACTIVITIES
Property, plant and equipment	-174.7	-166.2	-162.1
Cash used in disposition of assets	-3.4	-3.9	-2.3
Proceeds from disposition of assets	0.8	0.3	0.4
Affiliated-company property, plant and equipment transfers	0	10.8	0
NET CASH USED IN INVESTING ACTIVITIES	-177.3	-159	-164
FINANCING ACTIVITIES
Long-term debt issued, net of issuance costs	0	149	148.8
Long-term debt repaid	0	-42	-91.5
Change in notes payable to Questar	101.6	-58.4	23.4
Dividends paid	-27	-35.5	-33
NET CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES	74.6	13.1	47.7
Change in cash and cash equivalents	11	7.4	-3.6
Beginning cash and cash equivalents	8.8	1.4	5
Ending cash and cash equivalents	19.8	8.8	1.4
Supplemental Disclosure of Cash Paid (Received) During the Year for:
Interest	26.3	21	24.7
Income taxes	-13.2	-13.8	-20.4
Supplemental Disclosure of Noncash Investing and Financing Transaction:
Noncash reduction in notes payable to Questar due to equity contribution		90
Questar Pipeline [Member]
OPERATING ACTIVITIES
Net income	60.6	8.2	64.7
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	56.9	58.1	56.8
Deferred income taxes	12.6	-14.3	41.1
Abandonment and impairment	0	80.6	0
Share-based compensation	2.1	2	2
Net (gain) loss from asset sales	0.5	0	-2.7
(Income) from unconsolidated affiliate	-3.5	-3.7	-3.7
Distributions from unconsolidated affiliate	4.4	4.6	4.5
Other operating	0.5	0.5	0.4
Changes in operating assets and liabilities
Accounts receivable, including unbilled gas sales	-1.3	-8.5	-2.1
Inventories	-2.2	-0.9	0.2
Prepaid expenses and other	0.1	3.3	-3.3
Accounts payable and accrued expenses	-7.6	0.4	-3.4
Federal income taxes	-3.6	2.9	0
Current regulatory assets and liabilities	4.5	2.1	-0.9
NET CASH PROVIDED BY OPERATING ACTIVITIES	124	135.3	153.6
INVESTING ACTIVITIES
Property, plant and equipment	-59.4	-73.4	-60.6
Cash used in disposition of assets	-1.4	-1	-0.7
Proceeds from disposition of assets	0.2	0.1	4.3
Affiliated-company property, plant and equipment transfers	0	-10.7	0
NET CASH USED IN INVESTING ACTIVITIES	-60.6	-85	-57
FINANCING ACTIVITIES
Change in notes receivable from Questar	-10.7	9.3	-29.9
Dividends paid	-48	-64	-63
NET CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES	-58.7	-54.7	-92.9
Change in cash and cash equivalents	4.7	-4.4	3.7
Beginning cash and cash equivalents	2.7	7.1	3.4
Ending cash and cash equivalents	7.4	2.7	7.1
Supplemental Disclosure of Cash Paid (Received) During the Year for:
Interest	25.4	25.2	25.2
Income taxes	$32.40	$11	($3.80)

Summary_of_Significant_Account

Summary of Significant Accounting Policies (Notes)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Note 1 - Summary of Significant Accounting Policies
	A. Nature of Business
	Questar Corporation (Questar or the Company) is a Rockies-based integrated natural gas company with three principal complementary and wholly-owned lines of business:
	•	Questar Gas Company (Questar Gas) provides retail natural gas distribution in Utah, Wyoming and Idaho.
	•	Wexpro Company (Wexpro) develops and produces natural gas from cost-of-service reserves for Questar Gas customers.
	•	Questar Pipeline Company (Questar Pipeline) operates interstate natural gas pipelines and storage facilities in the western United States and provides other energy services.
	Questar is headquartered in Salt Lake City, Utah. Shares of Questar common stock trade on the New York Stock Exchange (NYSE:STR).
	B. Principles of Consolidation
	The Questar and Questar Pipeline consolidated financial statements contain the accounts of the parent companies and their majority-owned or controlled subsidiaries. The consolidated financial statements were prepared in accordance with U.S. generally accepted accounting principles (GAAP) and with the instructions for Annual Reports on Form 10-K and SEC Regulations S-X and S-K. All significant intercompany accounts and transactions have been eliminated in consolidation.
	C. Investment in Unconsolidated Affiliate
	Questar and Questar Pipeline use the equity method to account for an investment in an unconsolidated affiliate where they do not have control, but have significant influence. The investment in the unconsolidated affiliate on the Consolidated Balance Sheets equals Questar Pipeline's proportionate share of equity reported by the unconsolidated affiliate. The investment is assessed for possible impairment when events indicate that the fair value of the investment may be below Questar Pipeline's carrying value. When such a condition is deemed to be other-than-temporary, the carrying value of the investment is written down to its fair value, and the amount of the write-down is included in the determination of net income.
	White River Hub, LLC, a limited liability company and FERC-regulated transporter of natural gas, is the sole unconsolidated affiliate. Questar Pipeline owns 50% of White River Hub, LLC, and is the operator.
	D. Use of Estimates
	The preparation of financial statements and notes in conformity with GAAP requires that management formulate estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses and the disclosure of contingent assets and liabilities. The Company also incorporates estimates of proved developed and total proved gas and oil reserves in the calculation of depreciation, depletion and amortization rates of its gas and oil properties. Changes in estimated quantities of the Company's reserves could impact its reported financial results as well as disclosures regarding the quantities of proved gas and oil reserves. Actual results could differ from these estimates.
	E. Revenue Recognition
	Questar Gas
	Questar Gas records revenues in the period that gas is delivered, including gas delivered to residential and commercial customers but not billed as of the end of the accounting period. Unbilled gas deliveries are estimated for the period from the date meters are read to the end of the month. Approximately one-half month of revenue is estimated in any period. Gas costs and other variable costs are recorded on the same basis to ensure proper matching of revenues and expenses. Questar Gas's tariff allows for monthly adjustments to customer bills to approximate the effect of abnormal weather on non-gas revenues. The weather-normalization adjustment significantly reduces the impact of weather on gas-distribution earnings. The PSCU and PSCW approved a conservation enabling tariff (CET) to promote energy conservation. Under the CET, Questar Gas non-gas revenues are decoupled from the volume of gas used by customers. The tariff specifies an allowed monthly revenue per customer, with differences to be deferred and recovered from or refunded to customers through periodic rate adjustments. Rate adjustments occur every six months under the CET. The adjustments amortize deferred CET amounts over a 12-month period. These adjustments are limited to 5% of non-gas revenues.
	Questar Gas allows customers the option of paying an estimated fixed monthly bill throughout the year on a budget-billing program. The estimated payments are adjusted to actual usage annually. Amounts collected from customers under this program in excess of gas deliveries are recorded on the Consolidated Balance Sheets as customer advances. The budget-billing option does not impact revenue recognition. Questar Gas may collect revenues subject to possible refunds and establish reserves pending final orders from regulatory agencies.
	Wexpro
	Wexpro recognizes revenues in the period that services are provided or products are delivered. In accordance with the Wexpro agreements, production from the gas properties operated by Wexpro is delivered to Questar Gas at Wexpro's cost of providing this service, including an after-tax return on Wexpro's investment. Wexpro sells crude oil and NGL production from certain producing properties at market prices, with the revenues used to recover operating expenses and to provide Wexpro a return on its investment. Any operating income remaining after recovery of expenses and Wexpro's return on investment is divided between Questar Gas and Wexpro, with Wexpro retaining 46%. Amounts received by Questar Gas from the sharing of Wexpro's oil and NGL income are used to reduce natural gas costs to utility customers.
	Wexpro's investment base consists of its costs of acquired properties and commercial wells and related facilities, and is adjusted for working capital and reduced for deferred income taxes and accumulated depreciation, depletion and amortization. Property acquisition costs only pertain to properties that have been approved under the Wexpro II Agreement. Wexpro may collect revenues subject to possible refunds and establish reserves pending final calculation of the after-tax return on investment, which is adjusted annually.
	Revenues associated with the sale of gas, oil and NGL are accounted for using the sales method, whereby revenue is recognized as gas, oil and NGL are sold to purchasers. A liability is recorded to the extent that Wexpro has sold or delivered volumes in excess of its share of remaining gas and oil reserves in the underlying properties.
	Questar Pipeline
	Questar Pipeline and subsidiaries recognize revenues in the period that services are provided or products are delivered. The straight-fixed-variable rate design used by Questar Pipeline, which allows for recovery of substantially all fixed costs in the demand or reservation charge, reduces the earnings impact of volume changes on gas transportation and storage operations. Questar Pipeline may collect revenues subject to possible refunds and establish reserves pending final orders from regulatory agencies.
	F. Cost of Sales
	Questar Gas obtains the majority of its gas supply from Wexpro's cost-of-service production and pays Wexpro an operator service fee based on the terms of the Wexpro agreements. Questar Gas also obtains transportation and storage services from Questar Pipeline. These intercompany revenues and expenses are eliminated in the Questar Consolidated Statements of Income by reducing revenues and cost of sales. The underlying costs of Wexpro's production and Questar Pipeline's transportation and storage services are disclosed in other categories in the Consolidated Statements of Income, including operating and maintenance expense and depreciation, depletion and amortization expense. During the second and third quarters of the year, a significant portion of the natural gas from Wexpro production is injected into underground storage. This gas is withdrawn from storage as needed during the heating season in the first and fourth quarters. The cost of natural gas sold is credited with the value of natural gas as it is injected into storage and debited as it is withdrawn from storage. The reported balance in consolidated cost of sales may be a negative amount during the second and third quarters because of the entries to record injection of gas into storage and the elimination of intercompany transactions.
	The details of Questar's consolidated cost of sales are as follows:
		Year Ended December 31,
		2014			2013			2012
		(in millions)
	Questar Gas
	Gas purchases	$	136.5		$	186.6		$	104.2
	Operator service fee	349.7			294.6			274
	Transportation and storage	79.6			80.1			79.6
	Gathering	21			18.8			20.5
	Royalties	60.1			44.3			32
	Storage (injection) withdrawal, net	(1.1		)	(0.8		)	1.9
	Purchased-gas account adjustment	(45.8		)	22			16.1
	Other	4.8			5			5
	Total Questar Gas cost of natural gas sold	604.8			650.6			533.3
	Elimination of Questar Gas cost of natural gas sold - affiliated companies	(423.4		)	(370.9		)	(347.7		)
	Total Questar Gas cost of natural gas sold - unaffiliated parties	181.4			279.7			185.6
	Questar Pipeline
	Total Questar Pipeline cost of sales	4			6.1			6.7
	Other cost of sales	0.9			0.1			—
	Total cost of sales	$	186.3		$	285.9		$	192.3
	G. Regulation
	The Company applies the regulatory accounting principles to its rate-regulated businesses. Under these principles, the Company records regulatory assets and liabilities that would not be otherwise recorded under GAAP for non-rate-regulated entities. Regulatory assets and liabilities record probable future revenues or expenses associated with certain charges or credits that will be recovered from or refunded to customers through the rate-making process.
	Questar Gas accounts for purchased-gas costs in accordance with procedures authorized by the PSCU and the PSCW. Purchased-gas costs that are different from those provided for in present rates are accumulated and recovered or credited through future rate changes. Questar Gas may hedge a portion of its natural gas supply to mitigate price fluctuations for gas-distribution customers. The regulatory commissions allow Questar Gas to record periodic mark-to-market adjustments for commodity-price derivatives in the purchased-gas adjustment account. Questar did not have any commodity-price derivatives at December 31, 2014 or 2013. See Note 11 for a description and comparison of regulatory assets and liabilities as of December 31, 2014 and 2013.
	Wexpro manages and produces cost-of-service reserves for gas utility affiliate Questar Gas under the terms of the Wexpro agreements, comprehensive agreements with the states of Utah and Wyoming (see Note 10).
	Questar Gas is regulated by the PSCU and the PSCW. The Idaho Public Utilities Commission has contracted with the PSCU for rate oversight of Questar Gas operations in a small area of southeastern Idaho. Questar Pipeline is regulated by the FERC. These regulatory agencies establish rates for the sale, storage and transportation of natural gas. The regulatory agencies also regulate, among other things, the extension and enlargement or abandonment of jurisdictional natural gas facilities. Regulation is intended to permit the recovery, through rates, of the cost of service, including a return on investment.
	H. Cash and Cash Equivalents
	Cash equivalents consist principally of repurchase agreements with maturities of three months or less. In almost all cases, the repurchase agreements are highly liquid investments in overnight securities made through commercial bank accounts that result in available funds the next business day.
	I. Notes Payable to and Notes Receivable from Questar
	Notes payable to or receivable from Questar appearing in the financial statements and disclosures of Questar Gas and Questar Pipeline represent interest bearing demand notes for cash borrowed from Questar for use in operations or loaned to Questar until needed in operations. The funds are centrally managed by Questar. Amounts loaned to Questar earn an interest rate that is identical to the interest rate paid by the companies for borrowings from Questar.
	J. Property, Plant and Equipment
	Property, plant and equipment balances are stated at historical cost. Maintenance and repair costs are expensed as incurred.
	Cost-of-service gas and oil operations
	The successful efforts method of accounting is used for cost-of-service reserves developed and produced by Wexpro for gas utility affiliate Questar Gas. Cost-of-service reserves are properties for which the operations and return on investment are subject to the Wexpro agreements (see Note 10). Under the successful efforts method, Wexpro capitalizes the costs of acquiring leaseholds, drilling development wells, drilling successful exploratory wells, and purchasing related support equipment and facilities. Geological and geophysical studies and other exploratory activities are expensed as incurred. Costs of production and general corporate activities are expensed in the period incurred. A gain or loss is generally recognized on assets as they are retired from service.
	Contributions in aid of construction
	Customer contributions in aid of construction reduce plant unless the amounts are refundable to customers. Contributions for main-line extensions may be refundable to customers if additional customers connect to the main-line segment within five years. Refundable contributions are recorded as liabilities until refunded or the five-year period expires without additional customer connections. Amounts not refunded reduce plant. Capital expenditures in the Consolidated Statements of Cash Flows are reported net of non-refundable contributions. As a result of Questar Gas's recent Utah general rate case, effective March 1, 2014, the Company does not expect to record any new refundable customer contributions in aid of construction for Utah customers.
	Depreciation, depletion and amortization
	Major categories of fixed assets in gas distribution, transportation and storage operations are grouped together and depreciated using a straight-line method. Gains and losses on asset disposals are recorded as adjustments in accumulated depreciation. The Company has not capitalized future abandonment costs on a majority of its long-lived gas distribution and transportation assets due to a lack of a legal obligation to restore the area surrounding abandoned assets. In these cases, the regulatory agencies have opted to leave retired facilities in the ground undisturbed rather than excavate and dispose of the assets. Depreciation rates for Questar Gas and Questar Pipeline are established through rate proceedings.
	Capitalized costs of development wells and leaseholds are amortized on a field-by-field basis using the unit-of-production method and the estimated proved developed or total proved gas and oil reserves. Oil and NGL volumes are converted to natural gas equivalents using the ratio of one barrel of crude oil, condensate or NGL to 6,000 cubic feet of natural gas. The Company capitalizes an estimate of the fair value of future abandonment costs associated with cost-of-service reserves and depreciates these costs using a unit-of-production method.
The following represent average depreciation, depletion and amortization rates of the Company's capitalized costs:
	Year Ended December 31,
	2014			2013			2012
Questar Gas distribution plant	2.7		%	2.7		%	2.8		%
Cost-of-service gas and oil properties, per Mcfe	$	1.75		$	1.56		$	1.49
Questar Pipeline transportation, storage and other energy services	3.2		%	3.4		%	3.4		%
Questar Gas's depreciation rates include a component for the cost of plant removal. Accordingly, Questar Gas recognizes the cost of plant removal as depreciation expense. The related cost of removal accrual is reflected as a regulatory liability on the Questar Gas Balance Sheets (see Note 11). At the time property, plant and equipment is retired, removal expenses less salvage, are charged to the regulatory cost of plant removal liability.
K. Impairment of Long-Lived Assets
Proved gas and oil properties are evaluated on a field-by-field basis for potential impairment. Other properties are evaluated on a specific-asset basis or in groups of similar assets, as applicable. Impairment is indicated when a triggering event occurs and the sum of the estimated undiscounted future net cash flows of an evaluated asset is less than the asset's carrying value. Triggering events could include, but are not limited to, an impairment of gas and oil reserves caused by mechanical problems, faster-than-expected decline of reserves, lease-ownership issues, other-than-temporary decline in gas and oil prices, and changes in the utilization of pipeline assets. If impairment is indicated, fair value is estimated using a discounted cash flow approach that incorporates market interest rates or, if available, other market data. The amount of impairment loss recorded, if any, is the difference between the fair value of the asset and the current net book value. Cash flow estimates require forecasts and assumptions for many years into the future for a variety of factors, including commodity prices, commodity transportation rates and operating costs. Wexpro recorded a $2.0 million pre-tax abandonment and impairment charge for its share of the remaining investment in the Brady field in the second quarter of 2014. Questar Pipeline recorded an $80.6 million pre-tax impairment of the eastern segment of its Southern Trails Pipeline in the third quarter of 2013. See Note 16 for additional details. There were no impairments in 2012.
L. Goodwill
Goodwill represents the excess of the amount paid over the fair value of net assets acquired in a business combination, and is not subject to amortization. Goodwill and indefinite-lived intangible assets are tested for impairment at least once a year or when a triggering event occurs. The Company evaluates whether it is more likely than not that the carrying value of a reporting unit is greater than its fair value using events and circumstances such as economic conditions, industry changes, financial performance, etc. Fair value is measured using actively traded market values of other comparable companies in the same businesses. If the fair value of the reporting unit exceeds its carrying value then goodwill is considered not to be impaired. If the carrying value of the business unit is greater than the fair value, an impairment of goodwill is recognized equal to the excess of the carrying amount of goodwill over its fair value.
M. Capitalized Interest and Allowance for Funds Used During Construction
The Company capitalizes interest costs when applicable. The PSCU, PSCW and FERC require the capitalization of an allowance for funds used during construction (AFUDC) for rate-regulated plant and equipment. The Wexpro agreements require capitalization of AFUDC on cost-of-service gas and oil development projects. Amounts recorded in the Consolidated Statements of Income for the capitalization of AFUDC and interest costs are disclosed in the table below:
	Year Ended December 31,
	2014			2013			2012
	(in millions)
AFUDC (recorded as an increase in interest and other income)
Questar Gas	$	0.9		$	—		$	—
Wexpro	0.8			4.6			2.1
Questar Pipeline	1.1			1.7			0.2
Total AFUDC	$	2.8		$	6.3		$	2.3
Capitalized interest costs (recorded as a reduction of interest expense)
Questar Gas	$	0.5		$	0.2		$	0.1
Questar Pipeline	0.5			0.6			0.1
Total capitalized interest costs	$	1		$	0.8		$	0.2
N. Derivative Instruments and Hedging Activities
The Company may elect to designate a derivative instrument as a hedge of exposure to changes in fair value or cash flows. A derivative instrument qualifies as a hedge if all of the following tests are met:
•	The item to be hedged exposes the Company to market risk.
•	The derivative reduces the risk exposure and is designated as a hedge at the inception of the hedging relationship.
•	At the inception of the hedge and throughout the hedge period, there is a high correlation between changes in the fair value of the derivative instrument and the fair value of the underlying hedged item.
If the hedged exposure is a fair value exposure, the gain or loss on the derivative instrument is recognized in earnings in the period of the change together with the offsetting gain or loss from the change in fair value of the hedged item. If the hedged exposure is a cash flow exposure, the effective portion of the gain or loss on the derivative instrument is reported initially as a component of accumulated other comprehensive income (loss) (AOCI) and subsequently reclassified into earnings when the forecasted transaction affects earnings. Any amount excluded from the assessment of hedge effectiveness, as well as the ineffective portion of the gain or loss, is reported currently in earnings. When a derivative instrument is designated as a cash flow hedge of a forecasted transaction that becomes probable of not occurring, the gain or loss on the derivative is immediately reclassified into earnings from AOCI. See Note 6 for further discussion on derivatives and hedging.
O. Credit Risk
The Rocky Mountain region is the Company's primary market area. Exposure to credit risk may be affected by the concentration of customers in this region due to changes in economic or other conditions. Customers include individuals and numerous commercial and industrial enterprises that may react differently to changing conditions. Management believes that its credit-review procedures, loss reserves, customer deposits and collection procedures have adequately provided for usual and customary credit-related losses. Loss reserves are periodically reviewed for adequacy and may be established on a specific-case basis.
Bad debt expense associated with accounts receivable amounted to $1.7 million in 2014, $0.2 million in 2013 and $1.2 million in 2012. The allowance for bad debts was $1.7 million at December 31, 2014 and 2013. Questar Gas's retail gas operations account for a majority of the bad debt expense. Questar Gas estimates bad debt expense as a percentage of general-service revenues with periodic adjustments. Uncollected accounts are generally written off six months after gas is delivered and interest is no longer accrued. Questar Gas recovers bad debt costs related to the gas-cost portion of rates in its Utah operations through a purchased-gas adjustment to rates.
P. Asset Retirement Obligations
Questar records an asset retirement obligation (ARO) when there is a legal obligation associated with the retirement of a tangible long-lived asset. Questar's AROs apply primarily to abandonment costs associated with Wexpro gas and oil wells, production facilities and certain other properties. The Company has not capitalized future abandonment costs on a majority of its long-lived transportation and distribution assets because the Company does not have a legal obligation to restore the area surrounding abandoned assets. In these cases, the regulatory agencies have opted to leave retired facilities in the ground undisturbed rather than requiring the Company to excavate and dispose of the assets. The fair value of retirement costs is estimated by Company personnel based on abandonment costs of similar properties available to field operations and depreciated over the life of the related assets. Revisions to estimates result from material changes in the expected timing or amount of cash flows associated with AROs. Income or expense resulting from the settlement of ARO liabilities is included in net gain (loss) from asset sales on the Consolidated Statements of Income. The ARO liability is adjusted to present value each period through an accretion calculation using a credit-adjusted risk-free interest rate. See Note 4 for further discussion on AROs.
Q. Income Taxes
Questar and its subsidiaries file a consolidated federal income tax return. Questar Gas and Questar Pipeline account for income taxes on a separate return basis and record tax expenses and benefits as they are generated. Questar Gas and Questar Pipeline make payments to or receive payments from Questar for such tax expenses or benefits as they are generated on the consolidated income tax return. Deferred income taxes are recorded for the temporary differences arising between the book and tax carrying amounts of assets and liabilities. These differences create taxable or tax-deductible amounts for future periods. Questar Gas and Questar Pipeline use the deferral method to account for investment tax credits as required by regulatory commissions. The Company records interest earned on income tax refunds in interest and other income and records penalties and interest charged on tax deficiencies in interest expense.
Accounting standards for income taxes specify the accounting for uncertainty in income taxes by prescribing a minimum recognition threshold for a tax position to be reflected in the financial statements. If recognized, the tax benefit is measured as the largest amount of tax benefit that is more-likely-than-not to be realized upon ultimate settlement. Management has considered the amounts and the probabilities of the outcomes that could be realized upon ultimate settlement and believes that it is more-likely-than-not that the Company's recorded income tax benefits will be fully realized. There were no unrecognized tax benefits at the beginning or end of the years ended December 31, 2014, 2013 or 2012. The 2014 federal income tax return has not been filed.
In the 2012 tax year, Questar began participating in the IRS's Compliance Assurance Process (CAP). For the 2014 and 2015 tax years, Questar was accepted into the CAP Maintenance program. The CAP employs real-time resolution to improve federal tax compliance by resolving all or most tax positions prior to filing the related tax return. Successful conclusion of the CAP allows the IRS to achieve an acceptable level of assurance regarding the accuracy of the taxpayer's filed tax return and to eliminate or substantially reduce the need for a traditional examination. The CAP Maintenance program is administered by the IRS and indicates that the Company is a compliant taxpayer. The IRS has closed its review of all prior year tax returns.
Federal tax statutes allowed taxpayers a deduction of bonus depreciation related to capital expenditures of 50% in 2012, 2013, and 2014. The effects of bonus depreciation and other significant book/tax timing differences resulted in a net operating loss (NOL) carryforward for federal income tax purposes of $26.9 million at December 31, 2012. Questar utilized the entire December 31, 2012 NOL on its 2013 federal income tax return. See Note 8 for further discussion on income taxes.
R. Earnings Per Share
Basic earnings per share (EPS) is computed by dividing net income by the weighted-average number of common shares outstanding during the reporting period, which includes vested undistributed restricted stock units (RSUs). Diluted EPS includes the potential increase in the number of outstanding shares that could result from the exercise of in-the-money stock options, the vesting of RSUs with forfeitable dividend equivalents and the distribution of performance shares that are part of the Company's Long-term Stock Incentive Plan (LTSIP), less shares repurchased under the treasury stock method. Restricted shares and RSUs with nonforfeitable dividends or dividend equivalents are participating securities for the computation of basic earnings per share under the two-class method. The application of the two-class method has an insignificant impact on the calculation of Questar's basic and diluted EPS. See Note 2 for further discussion on EPS.
S. Share-Based Compensation
Questar may issue stock options, restricted shares, RSUs and performance shares to certain officers, employees and non-employee directors under the LTSIP. The Company uses the Black-Scholes-Merton mathematical model in estimating the fair value of stock options and the Monte Carlo simulation method in estimating the fair value of performance shares for accounting purposes. The granting of restricted shares and RSUs results in recognition of compensation cost measured at the grant-date market price. Questar uses an accelerated method in recognizing share-based compensation costs with graded vesting periods. See Note 12 for further discussion on share-based compensation.
T. Comprehensive Income
Beginning in 2013, the Company and its subsidiaries adopted accounting guidance issued in February 2013 that requires additional disclosures about reclassifications out of AOCI.
Comprehensive income, as reported on Questar's Consolidated Statements of Comprehensive Income, is the sum of net income as reported on the Questar Consolidated Statements of Income and net other comprehensive income (loss) (OCI) as reported on the Questar Consolidated Statements of Common Shareholders' Equity. OCI includes recognition of the under-funded position of pension and other postretirement benefit plans, interest rate cash flow hedges, changes in the fair value of long-term investment, and the related income taxes. Income or loss is recognized when the pension and other postretirement benefit (OPB) costs are accrued, as the Company records interest expense for hedged interest payments and when the long-term investment is sold or otherwise realized.
Comprehensive income, as reported on Questar Pipeline's Consolidated Statements of Comprehensive Income, is the sum of net income as reported on the Questar Pipeline Consolidated Statements of Income and net OCI as reported on the Questar Pipeline Consolidated Statements of Common Shareholder's Equity. OCI includes interest rate cash flow hedges and the related income taxes. Income or loss is recognized as the company records interest expense for hedged interest payments. See Note 3 for additional information related to OCI and AOCI.
U. Business Segments
Line of business information is presented according to senior management's basis for evaluating performance considering differences in the nature of products, services and regulation, among other factors. Certain intersegment sales include intercompany profit.
V. Recent Accounting Developments
In May 2014, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606) (ASU 2014-09), to supersede nearly all existing revenue recognition guidance under U.S. GAAP. The core principle of ASU 2014-09 is to recognize revenues when promised goods or services are transferred to customers in an amount that reflects the consideration that is expected to be received for those goods or services. ASU 2014-09 is effective for the Company beginning on January 1, 2017 using either of two methods: (i) retrospective application to each prior reporting period presented with the option to elect certain practical expedients as defined in the ASU; or (ii) retrospective application with the cumulative effect of initially applying the guidance recognized at the date of initial application and providing certain additional disclosures as defined in the ASU. Early adoption is not permitted. The Company is currently evaluating the ASU's impact on its financial position, results of operations and cash flows, as well as which transition approach it will take.
W. Reclassifications
Certain reclassifications were made to prior year financial statements and notes to conform to the 2014 presentation.
All dollar amounts in this Annual Report are in millions, except per-share information and where otherwise noted.

Earnings_Per_Share_Notes

Earnings Per Share (Notes)	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Earnings Per Share	Note 2 - Earnings Per Share
	Earnings Per Share
	A reconciliation of the components of basic and diluted shares used in the EPS calculation follows:
		Year Ended December 31,
		2014	2013	2012
		(in millions)
	Weighted-average basic common shares outstanding	175.8	175.4	176.5
	Potential number of shares issuable under the Company's LTSIP	0.3	0.6	1
	Weighted-average diluted common shares outstanding	176.1	176	177.5
	In the past three years, Questar had the ability to issue shares under the terms of the Dividend Reinvestment and Stock Purchase Plan, 401(k) Retirement Income Plan (see Note 13) and LTSIP (see Note 12).
	Dividend Reinvestment and Stock Purchase Plan
	The Dividend Reinvestment and Stock Purchase Plan allows shareholders to reinvest dividends or invest additional funds in Questar common stock. The Company can issue new shares or buy shares in the open market to meet shareholders' purchase requests. The Company bought shares in the open market to satisfy shareholders' purchases in 2014, 2013, and 2012 and no shares were issued. At December 31, 2014, there were 19,996,038 shares reserved and authorized for future issuance.

Accumulated_Other_Comprehensiv

Accumulated Other Comprehensive Income (Notes)	12 Months Ended
	Dec. 31, 2014
Other Comprehensive Income [Abstract]
Other Comprehensive Income	Note 3 - Accumulated Other Comprehensive Income
	Details of the changes in the components of AOCI, net of income taxes, as reported in Questar's Consolidated Statements of Common Shareholders' Equity, are shown in the tables below. The tables also disclose details of income taxes related to each component of OCI:
		Pension									OPB			Interest rate cash flow hedges			Long-term invest.			Total
		Actuarial loss			Prior service cost			Total			Actuarial loss
		(in millions)
	Balances at December 31, 2012	$	(202.4	)	$	(1.1	)	$	(203.5	)	$	(19.7	)	$	(23.2	)	$	0.1		$	(246.3	)
	OCI before reclassifications	123.4			—			123.4			12.4			—			(0.1		)	135.7
	Reclassified from AOCI(1)	28.9			1.1			30			3			0.5			—			33.5
	Income taxes
	OCI before reclassifications	(47.2		)	—			(47.2		)	(4.7		)	—			0.1			(51.8		)
	Reclassified from AOCI(2)	(11.1		)	(0.4		)	(11.5		)	(1.2		)	(0.2		)	—			(12.9		)
	Total income taxes	(58.3		)	(0.4		)	(58.7		)	(5.9		)	(0.2		)	0.1			(64.7		)
	Net other comprehensive income	94			0.7			94.7			9.5			0.3			—			104.5
	Balances at December 31, 2013	(108.4		)	(0.4		)	(108.8		)	(10.2		)	(22.9		)	0.1			(141.8		)
	OCI before reclassifications	(111.5		)	—			(111.5		)	(2.1		)	—			—			(113.6		)
	Reclassified from AOCI(1)	15.4			0.6			16			0.7			0.5			—			17.2
	Income taxes
	OCI before reclassifications	42.6			—			42.6			0.8			—			—			43.4
	Reclassified from AOCI(2)	(5.9		)	(0.2		)	(6.1		)	(0.3		)	(0.1		)	—			(6.5		)
	Total income taxes	36.7			(0.2		)	36.5			0.5			(0.1		)	—			36.9
	Net other comprehensive income (loss)	(59.4		)	0.4			(59.0		)	(0.9		)	0.4			—			(59.5		)
	Balances at December 31, 2014	$	(167.8	)	$	—		$	(167.8	)	$	(11.1	)	$	(22.5	)	$	0.1		$	(201.3	)
	(1) Interest rate cash flow hedge amounts are included in their entirety as charges to interest expense on the Consolidated Statements of Income.
	(2) Income tax reclassifications related to interest rate cash flow hedge amounts are included in their entirety as credits to income taxes on the Consolidated Statements of Income.
	Pension and other postretirement benefit AOCI reclassifications are included in the computation of net periodic pension and postretirement benefit costs. See Note 13 for additional details.
	Income taxes allocated to each component of Questar's consolidated OCI for the year ended December 31, 2012 are shown below. Expenses are in parentheses.
		Year Ended December 31,
		2012
		(in millions)
	Change in unrecognized actuarial loss	$	13.6
	Change in unrecognized prior service cost	(1.0		)
	Interest rate cash flow hedges	(0.2		)
	Change in fair value of long-term investment	(0.1		)
	Income taxes	$	12.3
	Disclosures regarding interest rate cash flow hedges, including income taxes and income statement reclassification effects, apply to Questar Pipeline. There were no significant changes in Questar Pipeline AOCI during 2012.

Asset_Retirement_Obligations_N

Asset Retirement Obligations (Notes)	12 Months Ended
	Dec. 31, 2014
Asset Retirement Obligation [Abstract]
Asset Retirement Obligation Disclosure	Note 4 - Asset Retirement Obligations
	Changes in Questar's AROs from the Consolidated Balance Sheets were as follows:
		Year Ended December 31,
		2014			2013
		(in millions)
	AROs at beginning of year	$	67.7		$	67.2
	Accretion	3.4			3.3
	Liabilities incurred	3.3			6
	Revisions in estimated cash flows	(2.7		)	(6.9		)
	Liabilities settled	(2.4		)	(1.9		)
	AROs at end of year	$	69.3		$	67.7
	Questar's consolidated AROs by line of business are summarized in the table below:
		December 31,
		2014			2013
		(in millions)
	Questar Gas	$	0.6		$	0.6
	Wexpro	66.3			64.9
	Questar Pipeline	2.3			2.2
	Other	0.1			—
	Total	$	69.3		$	67.7
	Wexpro collects from Questar Gas and deposits in trust certain funds related to estimated ARO costs. The funds are recorded as other noncurrent assets on the Consolidated Balance Sheets and are used to satisfy retirement obligations as the properties are abandoned. The accounting treatment of reclamation activities associated with AROs for properties administered under the Wexpro agreements is defined in a guideline letter between Wexpro and the Utah Division of Public Utilities and the staff of the PSCW.

Fair_Value_Measurements_Notes

Fair Value Measurements (Notes)	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Fair Value Measurements	Note 5 - Fair Value Measurements
	Questar complies with the accounting standards for fair value measurements and disclosures. These standards define fair value in applying GAAP, establish a framework for measuring fair value and require disclosures about fair value measurements. The standards establish a fair value hierarchy. Level 1 inputs are unadjusted quoted prices for identical assets or liabilities in active markets that the Company has the ability to access at the measurement date. Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. Level 3 inputs are unobservable inputs for the asset or liability. The Company had no assets or liabilities measured using Level 3 inputs at December 31, 2014 or 2013. Fair value accounting standards also apply to certain nonfinancial assets and liabilities that are measured at fair value on a nonrecurring basis. Questar did not have any assets or liabilities measured at fair value on a nonrecurring basis at December 31, 2014 or 2013.
	Questar primarily applies the market approach for recurring fair value measurements and maximizes its use of observable inputs and minimizes its use of unobservable inputs. Questar considers bid and ask prices for valuing the majority of its assets and liabilities measured and reported at fair value. In addition to using market data, Questar makes assumptions in valuing its assets and liabilities, including assumptions about the risks inherent in the inputs to the valuation technique.
	Questar
	The following table discloses the carrying amount, estimated fair value and level within the fair value hierarchy of certain financial instruments not disclosed in other notes to Questar's financial statements in this Annual Report:
		Hierarchy Level of Fair Value Estimates	Carrying Amount		Estimated Fair Value		Carrying Amount		Estimated Fair Value
			December 31, 2014				December 31, 2013
			(in millions)
	Financial assets
	Cash and cash equivalents	1	$	32	$	32	$	16	$	16
	Long-term investment	1	15.7		15.7		17.3		17.3
	Financial liabilities
	Short-term debt	1	347		347		276		276
	Long-term debt, including current portion	2	1,245.20		1,356.10		1,247.10		1,310.80
	The carrying amounts of cash and cash equivalents and short-term debt approximate fair value. The long-term investment is recorded at fair value and consists of money market and short-term bond index mutual funds representing funds held in Wexpro's trust (see Note 4). The fair value of the long-term investment is based on quoted prices for the underlying funds. The fair value of fixed-rate long-term debt is based on the discounted present value of cash flows using the Company's current credit risk-adjusted borrowing rates.
	The Questar Condensed Consolidated Balance Sheet included a nonrecurring fair value measurement at June 30, 2014 related to the impairment of Wexpro's investment in the Brady field. The asset's fair value of zero was based on Wexpro's assessment that the field had reached the end of its productive life and would no longer generate positive cash flows. This was a Level 3 fair value measurement because the inputs were unobservable. See Note 16 for additional information.
	Questar Gas
	The following table discloses the carrying amount, estimated fair value and level within the fair value hierarchy of certain financial instruments not disclosed in other notes to Questar Gas's financial statements in this Annual Report:
		Hierarchy Level of Fair Value Estimates	Carrying Amount		Estimated Fair Value		Carrying Amount		Estimated Fair Value
			December 31, 2014				December 31, 2013
			(in millions)
	Financial assets
	Cash and cash equivalents	1	$	19.8	$	19.8	$	8.8	$	8.8
	Financial liabilities
	Notes payable to Questar	1	119.3		119.3		17.7		17.7
	Long-term debt	2	534.5		607.2		534.5		568
	The carrying amounts of cash and cash equivalents approximate fair value. The carrying amounts of notes payable to Questar approximate fair value because of their short maturities and market-based interest rates. The fair value of fixed-rate long-term debt is based on the discounted present value of cash flows using Questar Gas's current credit risk-adjusted borrowing rates.
	Questar Pipeline
	The following table discloses the carrying amount, estimated fair value and level within the fair value hierarchy of certain financial instruments not disclosed in other notes to Questar Pipeline's financial statements in this Annual Report:
		Hierarchy Level of Fair Value Estimates	Carrying Amount		Estimated Fair Value		Carrying Amount		Estimated Fair Value
			December 31, 2014				December 31, 2013
			(in millions)
	Financial assets
	Cash and cash equivalents	1	$	7.4	$	7.4	$	2.7	$	2.7
	Notes receivable from Questar	1	40.1		40.1		29.4		29.4
	Financial liabilities
	Long-term debt, including current portion	2	458.8		495.5		458.9		486
	The carrying amounts of cash and cash equivalents approximate fair value. The carrying amounts of notes receivable from Questar approximate fair value because of their short maturities and market-based interest rates. The fair value of fixed-rate long-term debt is based on the discounted present value of cash flows using Questar Pipeline's current credit risk-adjusted borrowing rates.
	The Questar and Questar Pipeline Condensed Consolidated Balance Sheets included a nonrecurring fair value measurement at September 30, 2013 related to the impairment of the eastern segment of Questar Southern Trails Pipeline. The asset's fair value of zero was based on inputs that indicated overall negative future net cash flows. Questar Pipeline used a probability-weighted discounted cash flow analysis that included significant inputs such as Questar Pipeline's cost of capital and assumptions regarding future transportation rates and operating costs. This was a Level 3 fair value measurement because the inputs were unobservable. Based on this analysis, Questar Pipeline determined that the asset was impaired. See Note 16 for additional information.

Derivatives_and_Hedging_Notes

Derivatives and Hedging (Notes)	12 Months Ended
	Dec. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivatives and Hedging	Note 6 - Derivatives and Hedging
	Questar and its subsidiaries may enter into derivative instruments to manage exposure to changes in current and future market interest rates. In order to mitigate its exposure to changes in the fair value of its fixed-rate corporate debt resulting from changes in benchmark interest rates, in the second quarter of 2011 Questar executed a fixed-to-floating interest rate swap and converted $125.0 million of its 2.75% fixed-rate long-term debt to floating-rate debt. The 2.75% rate was swapped for a London Interbank Offered Rate (LIBOR)-based floating rate. Questar terminated and settled this hedge transaction in March 2012 for a deferred gain of $7.2 million, which is being amortized to interest expense through the maturity of the notes in 2016. Prior to its termination, this swap was accounted for as a fair value hedge under the accounting standards for derivatives and hedging.
	Questar Pipeline entered into forward starting swaps totaling $150.0 million in the second and third quarters of 2011 in anticipation of issuing $180.0 million of notes in December 2011. Settlement of these swaps required payments of $37.3 million because of declines in interest rates. These swaps qualified as cash flow hedges and the settlement payments are being amortized to interest expense over the 30-year life of the debt. See the Consolidated Statements of Comprehensive Income and Note 3 for details regarding reclassifications of AOCI related to deferred interest rate cash flow hedge losses to interest expense for 2014, 2013 and 2012.
	Reclassifications into earnings of amounts reported in AOCI will continue as interest expense is recorded for the hedged interest payments through maturity in 2041. Pre-tax net losses of $0.6 million are expected to be reclassified from AOCI to the Consolidated Statements of Income in the next 12 months. As of December 31, 2014, the Company was not hedging any exposure to variability in future cash flows of forecasted transactions. There were no derivative assets or liabilities outstanding at December 31, 2014 or 2013.

Debt_Notes

Debt (Notes)	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Debt	Note 7 - Debt
	The Company has a revolving credit facility with various banks to provide back-up credit liquidity support for its commercial paper program. Credit commitments under this revolving credit facility totaled $750.0 million at December 31, 2014, with no amounts borrowed. This revolving credit facility has interest-rate options generally below the prime interest rate and carries commitment fees on the unused balance. In April 2013, Questar amended and restated its revolving credit facility to increase its size from $500.0 million to $750.0 million and extend its maturity from August 31, 2016 to April 19, 2018. A covenant associated with the revolving credit facility stipulates that consolidated funded debt cannot exceed 70% of consolidated capitalization. The Company was in compliance with this covenant at December 31, 2014. The details of short-term debt are as follows:
		December 31,
		2014			2013
		(in millions)
	Commercial paper with various interest rates	$	347		$	276
	Weighted-average interest rate at end of year	0.23		%	0.27		%
	Availability under the revolving credit facility is reduced by outstanding commercial paper amounts. At December 31, 2014, Questar had net availability under the facility of $403.0 million.
	Questar centrally manages cash. Questar makes loans to Questar Gas and Questar Pipeline under a short-term borrowing arrangement. The interest rate paid on amounts borrowed is identical to the rate earned on amounts loaned under the arrangement. The rate is adjusted monthly based on prevailing short-term market interest rates.
	The following table details the notes payable to Questar from Questar Gas and the associated interest rates. There were no notes payable to Questar from Questar Pipeline at December 31, 2014 or 2013.
		December 31,
		2014			2013
		(in millions)
	Questar Gas
	Notes payable to Questar	$	119.3		$	17.7
	Interest rate at end of year	0.25		%	0.3		%
	All short- and long-term debt and the revolving credit facility are unsecured obligations and rank equally with all other unsecured liabilities. The terms of the Questar Corporation, Questar Gas and Questar Pipeline long-term debt obligations do not have dividend-payment restrictions.
	In December 2013, Questar Gas issued $90.0 million of 30-year Senior Notes at 4.78% and $60.0 million of 35-year Senior Notes at 4.83% in the private placement market. The proceeds of approximately $149.0 million, after deducting estimated issuance costs, were used to repay existing indebtedness and for general corporate purposes.
	The details of long-term debt are as follows:
		December 31,
		2014			2013
		(in millions)
	Questar Corporation
	2.75% Notes due 2016	$	250		$	250
	Questar Gas
	5.31% and 6.85% Medium-term Notes due 2017 and 2018	84.5			84.5
	6.30% Notes due 2018	50			50
	2.98% Notes due 2024	40			40
	3.28% Notes due 2027	110			110
	7.20% Notes due 2038	100			100
	4.78% Notes due 2043	90			90
	4.83% Notes due 2048	60			60
	Total Questar Gas long-term debt	534.5			534.5
	Questar Pipeline
	6.45% and 6.48% Medium-term Notes due 2015 and 2018	30.1			30.1
	5.83% Notes due 2018	250			250
	4.875% Notes due 2041	180			180
	Total Questar Pipeline long-term debt	460.1			460.1
	Total long-term debt outstanding	1,244.60			1,244.60
	Less current portion	(25.1		)	—
	Less unamortized debt discount	(1.9		)	(2.1		)
	Plus unamortized debt premium	0.5			0.7
	Plus fair value hedge adjustment	2			3.9
	Total long-term debt, less current portion	$	1,220.10		$	1,247.10
	The aggregate maturities of Questar Corporation, Questar Gas and Questar Pipeline long-term debt for the next five years are as follows:
		Questar Corporation			Questar			Questar Pipeline		Total
					Gas
		Years Ending December 31,
		(in millions)
	2015	$	—		$	—		$	25.1	$	25.1
	2016	250			—			—		250
	2017	—			14.5			—		14.5
	2018	—			120			255		375
	2019	—			—			—		—

Income_Taxes_Notes

Income Taxes (Notes)	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Income Taxes	Note 8 - Income Taxes
	Questar
	Details of Questar's income tax expense and deferred income taxes are provided in the following tables. The components of income tax expense were as follows:
		Year Ended December 31,
		2014				2013				2012
		(in millions)
	Federal
	Current	$	69.6			$	66.5			$	1.3
	Deferred	47.5				18.4				112
	State
	Current	6.9				10.7				—
	Deferred	2.1				5.9				3.6
	Deferred investment tax credits recognized	(0.2		)		(0.2		)		(0.4		)
	Total income tax expense	$	125.9			$	101.3			$	116.5
	The difference between the statutory federal income tax rate and the Company's effective income tax rate is explained as follows:
		Year Ended December 31,
		2014			2013			2012
	Federal income taxes statutory rate	35	%		35	%		35	%
	Increase (decrease) in rate as a result of:
	State income taxes, net of federal income tax benefit	1.6			4			1
	Tax benefits from dividends paid to employee stock plan	(1.0	)		(1.0	)		(0.5	)
	Other	0.1			0.6			—
	Effective income tax rate	35.7	%		38.6	%		35.5	%
	The combined federal and state income tax rate was lower in 2014 due to research and development and state tax credits and an increase in the domestic production deduction. The 2013 combined effective federal and state income tax rate increased due to the impairment of Southern Trails Pipeline. There was no state tax benefit recorded in association with the impairment charge because the Company has limited state operations for Southern Trails Pipeline. The effective combined federal and state income tax rate also increased in 2013 due to adjustments to estimated state income taxes for the consolidated Questar return that were in excess of state income taxes calculated on a separate return basis for the operating companies.
	Significant components of Questar's deferred income taxes were as follows:
		December 31,
		2014				2013
		(in millions)
	Deferred income taxes - noncurrent
	Deferred tax liabilities
	Property, plant and equipment	$	827			$	797.5
	Employee benefits	52				30.2
	Other	0.5				4.9
	Deferred tax liabilities - noncurrent	879.5				832.6
	Deferred tax assets
	Asset retirement obligations	22.5				22.7
	Pension and other postretirement benefits	110.8				73.7
	Deferred compensation	15.7				14.5
	Hedging activities	13.2				13.4
	State tax credits	3.3				3.4
	Valuation allowance	(1.6		)		(2.3		)
	Deferred tax assets, net of allowance - noncurrent	163.9				125.4
	Net deferred income tax liability - noncurrent	$	715.6			$	707.2
	Deferred income taxes - current
	Deferred tax assets - current	$	15.7			$	10.1
	Deferred tax liabilities - current	9.9				0.4
	Net deferred income tax asset - current	$	5.8			$	9.7
	Questar Gas
	Details of Questar Gas's income tax expense and deferred income taxes are provided in the following tables. The components of income tax expense were as follows:
		Year Ended December 31,
		2014				2013				2012
		(in millions)
	Federal
	Current	$	(11.9	)		$	6			$	(18.7	)
	Deferred	42.4				23.5				45.1
	State
	Current	(1.9		)		0.6				—
	Deferred	3.6				2				1.2
	Deferred investment tax credit recognized	(0.2		)		(0.2		)		(0.4		)
	Total income tax expense	$	32			$	31.9			$	27.2
	The difference between the statutory federal income tax rate and Questar Gas's effective income tax rate is explained as follows:
		Year Ended December 31,
		2014			2013			2012
	Federal income taxes statutory rate	35	%		35	%		35	%
Increase (decrease) in rate as a result of:
State income taxes, net of federal income tax benefit	1.3			2.1			1
Amortization of investment tax credits related to rate-regulated assets	(0.2	)		(0.3	)		(0.4	)
Other	0.6			0.9			1
Effective income tax rate	36.7	%		37.7	%		36.6	%
Significant components of Questar Gas's deferred income taxes were as follows:
	December 31,
	2014				2013
	(in millions)
Deferred income taxes - noncurrent
Deferred tax liabilities
Property, plant and equipment	$	354.4			$	322.6
Employee benefits	23.5				14
Other	0.5				4.9
Deferred tax liabilities - noncurrent	378.4				341.5
Deferred tax assets
Deferred compensation	0.9				0.8
Deferred tax assets - noncurrent	0.9				0.8
Net deferred income tax liability - noncurrent	$	377.5			$	340.7
Deferred income taxes - current
Deferred tax assets - current	$	3.6			$	3.3
Deferred tax liabilities - current	9.9				0.4
Net deferred income tax asset (liability) - current	$	(6.3	)		$	2.9
Questar Pipeline
Details of Questar Pipeline's income tax expense and deferred income taxes are provided in the following tables. The components of income tax expense were as follows:
	Year Ended December 31,
	2014				2013				2012
	(in millions)
Federal
Current	$	21.9			$	20.2			$	(5.4	)
Deferred	11.6				(15.2		)		40.4
State
Current	1				1.3				1.6
Deferred	1.1				0.7				0.7
Total income tax expense	$	35.6			$	7			$	37.3
The difference between the statutory federal income tax rate and Questar Pipeline's effective income tax rate is explained as follows:
	Year Ended December 31,
	2014			2013			2012
Federal income taxes statutory rate	35	%		35	%		35	%
Increase in rate as a result of:
State income taxes, net of federal income tax benefit	1.5			8.5			1.6
Other	0.5			2.6			—
Effective income tax rate	37	%		46.1	%		36.6	%
The 2013 combined effective federal and state income tax rate increased due to the impairment of Southern Trails Pipeline. There was no state tax benefit recorded in association with the impairment charge because the company has limited state operations for Southern Trails Pipeline.
Significant components of Questar Pipeline's deferred income taxes were as follows:
	December 31,
	2014				2013
	(in millions)
Deferred income taxes - noncurrent
Deferred tax liabilities
Property, plant and equipment	$	246.1			$	237.1
Employee benefits	10.8				7.4
Deferred tax liabilities - noncurrent	256.9				244.5
Deferred tax assets
Deferred compensation	2				1.8
Hedging activities	13.2				13.4
State tax credits	0.7				0.7
Valuation allowance	(0.4		)		—
Deferred tax assets, net of allowance - noncurrent	15.5				15.9
Net deferred income tax liability - noncurrent	$	241.4			$	228.6
Net deferred income tax asset - current	$	1.9			$	1.8

Contingencies_Commitments_and_

Contingencies, Commitments and Leases (Notes)	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Commitments, Contingencies and Leases	Note 9 - Contingencies, Commitments and Leases
	Contingencies
	Questar and each of its subsidiaries are involved in various commercial, environmental, and regulatory claims. Litigation and other legal proceedings arise in the ordinary course of business. Except as stated below concerning the QEP lawsuit, management does not believe any litigation or other legal proceedings individually or in the aggregate will have a material adverse effect on Questar's, Questar Gas's or Questar Pipeline's financial position, results of operations or cash flows.
	A liability is recorded for a loss contingency when its occurrence is probable and its amount can be reasonably estimated. If some amount within a range of possible outcomes appears to be a better estimate than any other amount within the range, that amount is recorded. Otherwise, the minimum amount in the range is recorded. Disclosures are provided for contingencies reasonably likely to occur, which would have a material adverse effect on Questar's, Questar Gas's or Questar Pipeline's financial position, results of operations or cash flows. Some of the claims involve highly complex issues relating to liability, damages and other matters subject to substantial uncertainties and, therefore, the probability of liability or an estimate of loss cannot be reasonably determined.
	Litigation
	On May 1, 2012, Questar Gas Company filed a legal action against QEP Field Services Company, a subsidiary of QEP Resources, Inc. The case, entitled Questar Gas Company v. QEP Field Services Company, was filed in the Third District Court in Salt Lake County, Utah. Questar Gas believes certain charges of QEP Field Services Company for gathering services exceed the amounts contemplated under a Gas Gathering Agreement, effective September 1, 1993, pertaining to certain gas produced by Wexpro Company under the Wexpro Agreement. Questar Gas is alleging breach of contract by QEP Field Services Company and is seeking an accounting, damages and a declaratory judgment relating to the services and charges under the Gas Gathering Agreement. The charges under the Gas Gathering Agreement are included in Questar Gas's rates as part of its purchased-gas costs. QEP Field Services Company filed an answer and counterclaim alleging that Questar Gas breached the Agreement by failing to allow QEP Field Services to gather and process gas from certain wells located in two fields in the state of Wyoming.
	On August 13, 2013, QEP Field Services Company assigned its interest in the Gas Gathering Agreement to QEPM Gathering I, LLC, a subsidiary of the general partner of the master limited partnership QEP Midstream Partners. Plaintiffs have filed an amended complaint naming QEP Midstream Partners, LP; QEP Midstream Partners GP, LLC; QEP Midstream Partners Operating, LLC; and QEPM Gathering I, LLC (QEP MLP Entities). QEP Field Services and Tesoro Logistics LP (Tesoro) entered into a Membership Interest Purchase Agreement dated October 19, 2014, to transfer QEP Field Services’ interest in the QEP MLP Entities and related assets and liabilities of QEP Field Services to Tesoro, including control of this legal action. Tesoro closed on the transaction for QEP’s midstream business on December 2, 2014.
	On December 2, 2014, the court issued a memorandum decision granting two motions for partial summary judgment for breach of contract filed by Questar Gas. The court found QEP Field Services Company breached the Gas Gathering Agreement by overcharging Questar Gas in its gathering rates. The court also denied two motions for partial summary judgment filed by QEP Field Services to reduce or limit contract damages. The court also denied cross-motions for partial summary judgment filed by both parties relating to another claim of breach of contract. The issues raised by the cross-motions, QEP Field Services’ counterclaim and damages on all claims are currently reserved for trial. No trial date has been set.
	While Questar Gas intends to vigorously pursue its legal rights, the claims and counterclaims involve complex legal issues and uncertainties that make it difficult to predict the outcome of the case and therefore management cannot determine at this time whether this litigation may have a material adverse effect on its financial position, results of operations or cash flows.
	In February 2015, a trial was held in the case of Rocky Mountain Resources and Robert N. Floyd v. QEP Energy Company and Wexpro Company, Ninth Judicial District, County of Sublette, State of Wyoming, Case No. 2011-7816. Plaintiffs allege they are entitled to a 4% overriding royalty interest (ORRI) in a so-called replacement state oil and gas lease ultimately assigned to Wexpro and QEP Energy Company (QEP) in the Pinedale Field. Wexpro and QEP believe the former state leases subject to the ORRI expired and a new lease was issued by the State of Wyoming unburdened by the 4% ORRI. A jury decision was reached on February 13, 2015, that awarded the Plaintiffs $14.1 million from Wexpro and $16.2 million from QEP. Wexpro and QEP plan to file an appeal of the case to the Wyoming Supreme Court. Wexpro has accrued its estimate of additional royalties in the case. Any additional royalties will be recovered from Questar Gas’s customers.
	Environmental Liabilities
	Questar incurs environmental remediation costs related to both owned and previously-owned facilities, including transmission and production facilities and manufactured gas plant sites.
	In 2014, Questar increased its environmental liability by $5.0 million to $5.4 million. This liability relates to a previously-owned chemical company and was increased due to changes in the costs of estimated future monitoring activities. The liability is based on undiscounted cash flows and is reported as a component of other noncurrent liabilities on the Consolidated Balance Sheets. The Company does not expect to incur significant additional losses related to this site.
	Commitments
	Questar Gas
	Currently, the majority of Questar Gas's natural gas supply is provided by cost-of-service reserves developed and produced by Wexpro. In 2014, Questar Gas purchased the remainder of its gas supply from multiple third parties under index-based or fixed-price contracts. Questar Gas has commitments to purchase gas for $18.3 million in 2015, $15.2 million in 2016, $18.2 million in 2017, and $21.2 million in 2018 and 2019 based on current prices. Generally, at the conclusion of the heating season and after a bid process, new agreements for the next heating season are put in place. Questar Gas bought natural gas under third-party purchase agreements amounting to $135.8 million in 2014, $186.5 million in 2013 and $104.1 million in 2012.
	In addition, Questar Gas stores gas during off-peak periods (typically during the summer) and withdraws gas from storage to meet peak gas demand (typically in the winter). The company has contracted for transportation and underground storage services with Questar Pipeline. Annual payments for these services amount to $74.4 million in 2015, $73.4 million in 2016, $46.7 million in 2017, $13.4 million in 2018, and $5.2 million in 2019. Questar Gas has third-party transportation and gathering commitments requiring yearly payments of $29.0 million in 2015 through 2017, $26.3 million in 2018, and $24.9 million in 2019.
	Leases
	The lease on the Company's former headquarters building ended on April 30, 2012. Rental expense on this long-term operating lease amounted to $1.5 million in 2012.
	In June 2010, Questar entered into a lease agreement for a new headquarters building. The lease term is 17 years and commenced on May 1, 2012. Rental payments under the lease escalate at a rate of 3% per year during the lease term. The lease agreement does not include bargain renewal periods or material rent holidays and is not subject to contingent rent or other unusual provisions. Questar accounts for this lease as a capital lease. Other property, plant and equipment on the Consolidated Balance Sheets includes $40.8 million under the capital lease as of December 31, 2014 and 2013. Other accumulated depreciation and amortization on the Consolidated Balance Sheets includes $6.7 million and $4.3 million under the capital lease as of December 31, 2014 and 2013, respectively. Amortization of the asset under the capital lease is included with depreciation, depletion and amortization on the Consolidated Statements of Income.
	Future minimum lease payments for the five years following 2014 and the years thereafter are shown in the table below. Also shown is the present value of minimum lease payments at December 31, 2014, which is reflected on the Consolidated Balance Sheets as current and noncurrent capital lease obligations of $1.0 million and $37.4 million, respectively.
		Years Ending December 31,
		(in millions)
	2015	$	3.1
	2016	3.5
	2017	3.6
	2018	3.7
	2019	3.8
	After 2019	41
	Total minimum lease payments	58.7
	Less: amount representing interest	(20.3		)
	Present value of minimum lease payments at December 31, 2014	$	38.4

Wexpro_and_Wexpro_II_Agreement

Wexpro and Wexpro II Agreements (Notes)	12 Months Ended
	Dec. 31, 2014
Wexpro Agreement [Abstract]
Wexpro and Wexpro II Agreements	Note 10 - Wexpro and Wexpro II Agreements
	Wexpro's operations are subject to the terms of the Wexpro and Wexpro II agreements. The original Wexpro Agreement was effective August 1, 1981, and sets forth the rights of Questar Gas to receive certain benefits from Wexpro's operations. The agreement was approved by the PSCU and PSCW (the Commissions) in 1981 and affirmed by the Supreme Court of Utah in 1983. The Wexpro II Agreement was modeled after the original Wexpro Agreement and allows for the addition of properties under the cost-of-service methodology for the benefit of Questar Gas customers. The Wexpro II Agreement was approved by the Commissions in 2013. The Utah Division of Public Utilities and the staff of the PSCW are entitled to review the performance of Questar Gas and Wexpro under the Wexpro agreements and have retained an independent certified public accountant and an independent petroleum engineer to monitor the performance of the agreements. Major provisions of the agreements are as follows:
	a. Wexpro conducts gas-development drilling on productive gas properties, including properties acquired and approved for inclusion in the Wexpro II Agreement, and bears any costs of dry holes. Natural gas produced from successful drilling on these properties is delivered to Questar Gas. Wexpro is reimbursed for the costs of producing the natural gas plus a return on its investment in successful wells. The after-tax return allowed Wexpro is adjusted annually and is currently 20.0%.
	b. Wexpro operates certain natural gas properties for Questar Gas. Wexpro is reimbursed for its costs of operating these properties, including a rate of return on any investment it makes. This after-tax rate of return is adjusted annually and is currently 12.0%.
	c. Wexpro conducts developmental-oil drilling on productive oil properties and bears any costs of dry holes. Oil and NGL produced from these properties is sold at market prices, with the revenues used to recover operating expenses and to give Wexpro a return on its investment in successful wells. The after-tax rate of return is adjusted annually and is currently 17.0%. Any operating income remaining after recovery of expenses and Wexpro's return on investment is divided between Wexpro and Questar Gas, with Wexpro retaining 46% and Questar Gas retaining 54%.
	d. Crude oil and NGL production from certain oil-producing properties is sold at market prices, with the revenues used to recover operating expenses and to provide Wexpro a return on its investment. The after-tax rate of return on investments in these properties is adjusted annually and is currently 12.0%. Any operating income remaining after recovery of expenses and Wexpro's return on investment is divided between Wexpro and Questar Gas, with Wexpro retaining 46%.
	e. Amounts received by Questar Gas from the sharing of Wexpro's oil and NGL income are used to reduce natural gas costs to utility customers.
	f. Acquired natural gas production from properties approved by the Commissions for inclusion in the Wexpro II Agreement is delivered to Questar Gas. Wexpro is reimbursed for the costs of producing the natural gas plus a return on its acquisition investment. The after-tax return allowed Wexpro is adjusted periodically and is currently 7.6%.
	g. Wexpro's return on investment base is determined based on authorized returns from a group of rate-regulated companies plus an 8% risk premium for natural gas development drilling costs and a 5% risk premium for oil development drilling costs. The authorized returns for this group of companies have declined in recent years, resulting in lower returns on investment base for Wexpro. Wexpro's return on investment base for Wexpro II property acquisition costs is based on Questar Gas's approved cost of capital.
	Wexpro's net investment base, yearly average rate of return, and oil and NGL income shared with Questar Gas are shown in the table below:
		Year Ended December 31,
		2014			2013			2012
	Wexpro net investment base at December 31, (in millions)	$	649		$	589.7		$	531.1
	Average annual rate of return (after tax)	17.9		%	19.7		%	19.9		%
	Oil and NGL income sharing (in millions)	$	—		$	0.6		$	2.5
	The lower return in 2014 was due primarily to the inclusion of the Trail acquisition (see Note 17) in the Wexpro II Agreement. The Wexpro II portion of the investment base earns the lower return described in item f., above.
	Trail Acquisition Stipulation
	In the first quarter of 2014, the Commissions approved a stipulation for inclusion of the Trail acquisition in the Wexpro II Agreement. As part of this stipulation, Wexpro agreed to manage the combined production from the original Wexpro properties and the Trail acquisition to 65% of Questar Gas's annual forecasted demand. Beginning in June 2015 through May 2016 and for each subsequent 12-month period, if the combined annual production exceeds 65% of the forecasted demand and the cost-of-service price is greater than the Questar Gas purchased-gas price, an amount equal to the excess production times the excess price will be credited back to Questar Gas customers. Wexpro may also sell production to manage the 65% level and credit back to Questar Gas customers the higher of market price or the cost-of-service price times the sales volumes.

Rate_Regulation_Notes

Rate Regulation (Notes)	12 Months Ended
	Dec. 31, 2014
Regulated Operations [Abstract]
Rate Regulation	Note 11 - Rate Regulation
	The following table details regulatory assets and liabilities:
		December 31, 2014				December 31, 2013
		Current		Noncurrent		Current		Noncurrent
		(in millions)
	Regulatory assets:
	Questar Gas
	Purchased-gas adjustment	$	39.2	$	—	$	—	$	—
	Demand-side management	—		—		11.2		—
	Contract withholding	13.6		—		8.1		—
	Deferred cost-of-service gas charges	25.5		9.3		10.9		2
	Cost of reacquired debt	—		4.3		—		4.8
	Pipeline integrity costs	—		7.7		—		9.3
	Total Questar Gas regulatory assets	78.3		21.3		30.2		16.1
	Questar Pipeline
	Gas imbalance	1.2		—		5.3		—
	Revenue sharing	0.1		—		0.3		—
	Cost of reacquired debt	—		2.2		—		2.6
	Income taxes recoverable from customers	—		0.3		—		0.6
	Other	—		1.2		—		1.2
	Total Questar Pipeline regulatory assets	1.3		3.7		5.6		4.4
	Total regulatory assets	$	79.6	$	25	$	35.8	$	20.5
	Regulatory liabilities:
	Questar Gas
	Purchased-gas adjustment	$	—	$	—	$	7.4	$	—
	Demand-side management	0.3		—		—		—
	CET	12.1		—		3.3		—
	Cost of plant removal	—		60.7		—		52.7
	Income taxes refundable to customers	—		0.2		—		0.3
	Other	0.1		—		0.4		—
	Total Questar Gas regulatory liabilities	12.5		60.9		11.1		53
	Questar Pipeline
	Gas imbalance	0.8		—		3		—
	Revenue sharing	0.1		—		—		—
	Postretirement medical	—		9		—		8.2
	Total Questar Pipeline regulatory liabilities	0.9		9		3		8.2
	Total regulatory liabilities	$	13.4	$	69.9	$	14.1	$	61.2
	Questar Gas and Questar Pipeline record regulatory assets and liabilities. They recover the costs of assets but do not generally receive a return on these assets.
	Following is a description of Questar Gas's regulatory assets and liabilities:
	-	Purchased-gas costs that are different from those provided for in present rates are accumulated and recovered or credited through future rate changes.
	-	The demand-side management program relates to funds expended for promoting the conservation of natural gas through advertising, rebates for efficient homes and appliances, and home energy audits. Costs are recovered from customers through periodic rate adjustments. Costs incurred in excess of recoveries result in an asset; recoveries in excess of costs incurred result in a liability.
	- Questar Gas recorded a regulatory asset for a disputed amount withheld from a supplier of gathering services. The amount withheld will be recovered from customers if it is determined that Questar Gas is required to pay the supplier.
	- Operating and maintenance, depreciation, depletion and amortization, production taxes and royalties on cost-of-service gas production are recorded when the gas is produced and recovered from customers on a delayed basis, generally within 12 months.
	-	Certain cost-of-service gas charges are recovered over a period greater than 12 months. These include a regulatory asset that represents future expenses related to abandonment of Wexpro-operated gas and oil wells. The regulatory asset is reduced over an 18-year period following an amortization schedule that commenced January 1, 2003, or as cash is paid to plug and abandon wells. Noncurrent cost-of-service gas charges also include amounts for production imbalances that will be recovered from customers at the end of the related gas wells' useful lives.
	-	Gains and losses on the reacquisition of debt by rate-regulated companies are deferred and amortized as interest expense over the would-be remaining life of the reacquired debt. The reacquired debt costs had a weighted-average life of approximately 8 years as of December 31, 2014.
	-	The costs of complying with pipeline-integrity regulations are recovered in rates subject to a PSCU order. Questar Gas is allowed to recover $7.0 million per year. Costs incurred in excess of this amount will be recovered in future rate changes.
	-	The CET liability represents actual revenues received that are in excess of the allowed revenues. These amounts are refunded through periodic rate adjustments.
	- Cost of plant removal represents asset retirement costs recovered from customers for other than legal obligations.
	-	Income taxes refundable to customers arise from adjustments to deferred taxes, refunded over the life of the related property, plant and equipment.
	Following is a description of Questar Pipeline's regulatory assets and liabilities:
	-	Regulatory assets and liabilities for gas imbalances, fuel over- or under-recovered and sharing interruptible revenues with customers.
	-	Gains and losses on the reacquisition of debt are deferred and amortized as interest expense over the would-be remaining life of the reacquired debt. The reacquired debt costs had a weighted-average life of approximately 6 years as of December 31, 2014.
	-	Income taxes recoverable from customers arise from adjustments to deferred taxes, recovered over the life of the related property, plant and equipment.
	-	A regulatory liability for the collection of postretirement medical costs allowed in rates in excess of actual charges.
	Rate Changes
	Questar Gas is authorized to earn a return on equity of 9.85% in Utah and 9.16% in Wyoming. Effective March 1, 2014, Questar Gas increased its rates in Utah by $7.6 million annually as a result of a general rate case filed in Utah in July 2013. The order in this rate case authorized an allowed return on equity of 9.85%. In December 2014, Questar Gas held hearings on a general rate case in Wyoming. At the hearings the PSCW ordered an increase in annualized revenues of $1.5 million and an authorized return on equity of 9.5%. The change in rates is expected to be effective March 1, 2015.

ShareBased_Compensation_Notes

Share-Based Compensation (Notes)	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-Based Compensation	Note 12 - Share-Based Compensation
	Questar may issue stock options, restricted shares, RSUs and performance shares to certain officers, employees and non-employee directors under the LTSIP. Questar recognizes expense over time as the stock options, restricted shares, RSUs and performance shares vest. Total share-based compensation expense amounted to $12.4 million in 2014 compared to $10.2 million in 2013 and $9.9 million in 2012. Cash flow from income tax benefits in excess of recognized compensation expense amounted to $2.2 million in 2014 and $14.0 million in 2013. There were no cash flows from income tax benefits in excess of recognized compensation expense in 2012. At December 31, 2014, there were 4,053,586 shares available for future grant. The following disclosures under the headings Stock Options, Restricted Shares, Restricted Stock Units and Performance Shares include all awards granted to officers, employees and non-employee directors of Questar and its affiliates. The disclosures under the heading Questar Gas and Questar Pipeline describe the subset of total awards granted to officers and employees of those companies.
	Stock Options
	Stock options have terms ranging from five to ten years, with a majority issued with a seven- to ten-year term. Options generally vest in three or four equal, annual installments. There were no unvested stock options at December 31, 2014 or 2013. No stock options have been granted since 2010 and no stock options were forfeited in 2014.
	Stock option transactions and balances under the terms of the LTSIP are summarized below:
		Options Outstanding			Exercise Price Range								Weighted-Average Exercise Price
	Balance at December 31, 2013	717,840			$	7.84		-	$	17.35			$	11.84
	Exercised	(282,333	)		7.84			-	13.24				10.88
	Balance at December 31, 2014	435,507			$	11.4		-	$	17.35			$	12.46
	Options Outstanding															Options Exercisable
	Range of exercise prices								Number outstanding at Dec. 31, 2014			Weighted-average remaining term in years		Weighted-average exercise price		Number exercisable at Dec. 31, 2014	Weighted-average exercise price
	$	11.4			$	11.4			239,174			1.2		$	11.4	239,174	$	11.4
	$	13.1		-	17.35				196,333			2		13.75		196,333	13.75
									435,507			1.5		$	12.46	435,507	$	12.46
	As of December 31, 2014, the aggregate intrinsic value of outstanding and exercisable options was $5.6 million.
	Certain officers, employees and non-employee directors of former subsidiary QEP Resources held 245,332 Questar stock options with a weighted-average exercise price of $11.34 per share and a weighted-average remaining life of 1.4 years at December 31, 2014.
	Restricted Shares
	Restricted shares are valued at the grant-date market price and amortized to expense over the vesting period. Most restricted share grants vest in equal installments over a three-year period from the grant date. Unvested restricted shares have voting and dividend rights; however, sale or transfer is restricted. The weighted-average remaining vesting period of unvested restricted shares at December 31, 2014, was 2 months. No restricted shares were granted in 2014. Transactions involving restricted shares under the terms of the LTSIP for the year ended December 31, 2014, are summarized below:
		Restricted Shares Outstanding			Price Range								Weighted- Average Price
	Balance at December 31, 2013	392,327			$	15.22		-	$	21.53			$	18.28
	Vested	(289,312	)		15.22			-	21.53				17.88
	Forfeited	(661	)		19.39			-	20.9				20
	Balance at December 31, 2014	102,354			$	17.97		-	$	21.53			$	19.41
	Restricted Stock Units
	Questar may grant RSUs to certain of its officers, employees and non-employee directors under the LTSIP. RSUs are valued at the grant-date market price and amortized to expense over the vesting period. RSU grants typically vest in equal installments over a three-year period from the grant date. Certain grants vest in a single installment after a specified period. RSUs do not have voting rights until shares are distributed, but they do have dividend equivalent rights. Most RSU dividend equivalents are paid in cash quarterly and vest immediately. One share of Questar common stock will be distributed for each RSU at the time of vesting. The weighted-average remaining vesting period of unvested RSUs at December 31, 2014, was 12 months.
	Transactions involving RSUs under the terms of the LTSIP for the year ended December 31, 2014, are summarized below:
		RSUs Outstanding			Price Range								Weighted-Average Price
	Balance at December 31, 2013	316,268			$	22.17		-	$	25.44			$	23.62
	Granted	351,066			22.14			-	24.7				23.59
	Vested	(131,007	)		22.17			-	25.44				23.64
	Forfeited	(10,225	)		23.6			-	23.62				23.6
	Balance at December 31, 2014	526,102			$	22.14		-	$	24.7			$	23.6
	Questar may grant RSUs with deferred share distributions (deferred RSUs) to certain of its non-employee directors under the LTSIP. Grants of deferred RSUs typically vest in a single installment over a one-year period from the grant date. Dividend equivalents on deferred RSUs accrue quarterly and are subject to the vesting, distribution and voting conditions of the underlying award. One share of Questar common stock will be distributed for each vested deferred RSU (including accrued reinvested dividend equivalents) at the time of the grantees' separation from service. At December 31, 2014, Questar's outstanding deferred RSUs totaled 21,860 with a weighted-average price of $23.61 per share. The weighted-average remaining vesting period of unvested deferred RSUs at December 31, 2014, was 2 months.
	Performance Shares
	Questar may grant performance shares to certain of its officers under the terms of the LTSIP. The awards are designed to motivate and reward these officers for long-term Company performance and provide an incentive for them to remain with the Company. The target number of performance shares for each officer is subject to a payout adjustment multiplier ranging from 0.00 to 3.00 based on the Company's total shareholder return relative to a specified peer group of companies over a three-year performance period. Each three-year performance period commences at the beginning of the year of grant. Distributions of performance shares, if any, take place in the quarter following the conclusion of the performance period, so long as such officer was employed by the Company or its affiliates as of the last day of the performance period.
	The Company uses the Monte Carlo simulation method to estimate the grant-date fair value of performance share awards. Fair value estimates rely upon subjective assumptions used in the mathematical model and may not be representative of future results. The estimated fair value of performance shares granted and major assumptions used in the simulation at the date of grant are listed below:
		2014				2013				2012
		Performance Share Input Variables
	Fair value of performance shares at grant date	$	31.07			$	39.62			$	25.42
	Risk-free interest rate	0.67		%		0.4		%		0.35		%
	Expected price volatility	18.6		%		19		%		22		%
	Expected dividend yield	3.05		%		2.88		%		3.35		%
	Expected life in years	2.9				2.9				2.9
	For performance shares granted in 2012 and 2013, half of any award will be distributed in shares of Questar common stock and half in cash. Subsequent awards will be distributed in shares, with no cash component. For share-settled performance share awards, the grant-date fair value of the awards is amortized to expense over the vesting period. The liability associated with awards to be settled in cash is adjusted to its estimated fair value through earnings on a quarterly basis. Equity- and liability-based performance share compensation expense amounted to $3.8 million in 2014, $2.0 million in 2013 and $1.8 million in 2012. The weighted-average remaining vesting period of unvested performance shares at December 31, 2014, was 18 months.
	Transactions involving performance shares under the terms of the LTSIP for the year ended December 31, 2014, are summarized below:
		Target Number of Performance Shares Outstanding			Grant-Date Fair Value Range								Weighted-Average Grant-Date Fair Value
	Balance at December 31, 2013	387,879			$	18.23		-	$	39.62			$	27.61
	Granted	139,611			31.07				31.07				31.07
	Payout adjustment(1)	(76,657	)		18.23				18.23				18.23
	Distributed(1)	(51,104	)		18.23				18.23				18.23
	Forfeited	(8,480	)		25.42			-	39.62				33.33
	Balance at December 31, 2014(2),(3)	391,249			$	25.42		-	$	39.62			$	31.78
	(1) Actual shares and cash distributed were determined by multiplying the target shares by 0.40 to reflect Questar's final total shareholder return ranking relative to the specified peer companies for the performance period from January 1, 2011 through December 31, 2013. The total amount paid for the cash half of the distribution was $0.6 million.
	(2) The total undistributed balance consists of 262,979 of share-settled target shares and 128,270 of cash-settled target shares.
	(3) The total undistributed balance includes 134,778 vested target shares that are expected to be distributed in the first quarter of 2015 using a payout adjustment multiplier of 1.00. The multiplier reflects Questar's final total shareholder return ranking relative to the specified peer companies for the performance period from January 1, 2012 through December 31, 2014. The distribution will be half in shares, half in cash. These target shares are included in the amounts described in note (2), immediately above.
	As of December 31, 2014, the total unrecognized compensation cost related to outstanding stock options, restricted shares, RSUs and performance shares was $8.8 million, which the Company expects to recognize over a weighted-average period of 12 months.
	Questar Gas and Questar Pipeline
	Questar may issue stock options, restricted shares, RSUs and performance shares to certain officers and employees of Questar Gas and Questar Pipeline under the LTSIP. Questar Gas and Questar Pipeline recognize expense over time as the stock options, restricted shares, RSUs and performance shares vest. Questar Gas share-based compensation expense amounted to $1.6 million in 2014 compared with $1.4 million in 2013 and $1.2 million in 2012. Questar Pipeline share-based compensation expense amounted to $2.1 million in 2014 compared with $2.0 million in 2013 and 2012.
	The following table summarizes the stock options held under the LTSIP by Questar Pipeline officers and employees at December 31, 2014. There were no stock options held under the LTSIP by Questar Gas officers or employees at December 31, 2014.
	Options Outstanding															Options Exercisable
	Range of exercise prices								Number outstanding at Dec. 31, 2014			Weighted-average remaining term in years		Weighted-average exercise price		Number exercisable at Dec. 31, 2014	Weighted-average exercise price
	Questar Pipeline
	$	11.4		-	$	13.1			58,933			1.5		$	11.95	58,933	$	11.95
	The following table summarizes the restricted shares held under the LTSIP by Questar Gas and Questar Pipeline officers and employees at December 31, 2014. The weighted-average remaining vesting periods of unvested restricted shares at December 31, 2014, for Questar Gas and Questar Pipeline were 2 months and 3 months, respectively.
	Restricted			Price Range								Weighted-Average Price
	Shares Outstanding
Questar Gas	12,635			$	19.39			$	19.39			$	19.39
Questar Pipeline	25,269			$	19.39		-	$	19.56			$	19.42
The following table summarizes the RSUs held under the LTSIP by Questar Gas and Questar Pipeline officers and employees at December 31, 2014. The weighted-average remaining vesting period of unvested RSUs at December 31, 2014, for Questar Gas and Questar Pipeline was 12 months.
	RSUs Outstanding			Price Range								Weighted-Average Price
Questar Gas	82,351			$	23.09		-	$	23.62			$	23.61
Questar Pipeline	95,841			$	23.09		-	$	23.62			$	23.59
The following table summarizes the target number of performance shares held under the LTSIP by Questar Gas and Questar Pipeline officers at December 31, 2014. The weighted-average remaining vesting period of unvested performance shares at December 31, 2014, for Questar Gas and Questar Pipeline was 18 months.
	Target Number of Performance Shares Outstanding			Grant-Date Fair Value Range								Weighted-Average Grant-Date Fair Value
Questar Gas	33,826			$	25.42		-	$	39.62			$	31.73
Questar Pipeline	59,956			$	25.42		-	$	39.62			$	31.59

Employee_Benefits_Notes

Employee Benefits (Notes)	12 Months Ended
	Dec. 31, 2014
Compensation and Retirement Disclosure [Abstract]
Employee Benefits	Note 13 - Employee Benefits
	Defined Benefit Pension Plans and Other Postretirement Benefits
	The Company has a noncontributory defined benefit pension plan covering a majority of its employees and postretirement medical and life insurance plans providing coverage to less than half of its employees. Employees hired or rehired after June 30, 2010 are not eligible for the noncontributory defined benefit pension plan and employees hired or rehired after December 31, 1996 are not eligible for the postretirement medical plan and are not eligible to receive basic life coverage once they retire.
	Questar funds a trust for Employee Retirement Income Security Act (ERISA)-qualified pension and other postretirement benefit obligations to pay benefits currently due and to build asset balances over time to pay future obligations. Questar is subject to and complies with minimum-required and maximum-allowed annual contribution levels mandated by ERISA and by the Internal Revenue Code. Subject to these limitations, the Company seeks to fund the qualified pension plan in amounts that are at a minimum equal to the yearly net cost. The Company also has a nonqualified pension plan that covers a group of management employees in addition to the qualified pension plan. The nonqualified pension plan provides for defined benefit payments upon retirement of the management employee, or to the spouse upon death of the management employee, above the benefit limit defined by the IRS for the qualified plan. The nonqualified pension plan is unfunded; benefits are paid from the Company's general funds. The Company commingles postretirement medical plan assets with those of the ERISA-qualified pension plan as permitted by section 401(h) of the Internal Revenue Code.
	The Company's Employee Benefits Committee (EBC) has oversight over investment of qualified pension and other postretirement benefit plan assets. The EBC uses a third-party consultant to assist in setting targeted policy ranges for the allocation of assets among various investment categories. The EBC allocates qualified pension, postretirement medical and postretirement life plan assets among broad asset categories and reviews the asset allocation at least annually. Asset allocation decisions consider risk and return, future benefit requirements, participant growth and other expected cash flows. The EBC seeks investment returns consistent with reasonable and prudent levels of liquidity and risk.
	The EBC uses asset-mix guidelines that include permissible ranges for each asset category, return objectives for each asset group and the desired level of diversification and liquidity. These guidelines change from time to time based on the EBC's ongoing evaluation of each plan's risk tolerance. The EBC estimates an expected overall long-term rate of return on assets by weighting expected returns of each asset class by its targeted asset allocation percentage. Expected return estimates are developed from analysis of past performance and forecasts of long-term return expectations by third parties.
	Qualified pension and other postretirement benefit plan assets were invested as follows:
		Actual Allocation						Policy Range
		December 31,						December 31,
		2014			2013			2014					2013
	Total domestic equity securities	38	%		38	%		35	%	-	45	%	35	%	-	45	%
	Foreign equity securities
	Developed market foreign equity securities	18	%		20	%
	Emerging market foreign equity securities	4	%		5	%
	Total foreign securities	22	%		25	%		25	%	-	35	%	25	%	-	35	%
	Debt securities
	Investment-grade intermediate-term debt	7	%		7	%
	Investment-grade long-term debt	14	%		11	%
	Below-investment-grade debt	9	%		9	%
	Total debt securities	30	%		27	%		25	%	-	35	%	25	%	-	35	%
	Inflation protection securities	9	%		9	%		—	%	-	10	%	—	%	-	10	%
	Cash and short-term investments	1	%		1	%		—	%	-	3	%	—	%	-	3	%
	At December 31, 2014, domestic equity assets were invested in a passive total stock market index fund that invests in a diversified portfolio of stocks representative of the whole U.S. stock market and an S&P 500 index fund. Developed market foreign equity assets were invested in funds that hold a diversified portfolio of common stocks of corporations in developed countries outside the United States. These investments are benchmarked against the Morgan Stanley Capital International Europe Australasia and Far East (MSCI EAFE) index. Emerging market foreign equity assets are invested in funds that hold a diversified portfolio of common stocks of corporations in emerging countries outside the United States and are benchmarked against the MSCI Emerging Markets index.
	Investment-grade intermediate-term debt assets are invested in funds holding a diversified portfolio of debt of governments, corporations and mortgage borrowers with average maturities of 5 to 10 years and investment-grade credit ratings. The investments are benchmarked against the Barclays Capital Aggregate Bond index. Investment-grade long-term debt assets are invested in a diversified portfolio of debt of governments, corporations and mortgage borrowers with an average maturity of more than 10 years and investment-grade credit ratings. These assets are benchmarked against the Barclays Capital Government/Credit Bond index. Below-investment-grade debt assets are invested in a fund holding a diversified portfolio of debt securities of corporations with an average maturity up to 10 years and below-investment-grade credit ratings. This investment is benchmarked against the Merrill Lynch High Yield II Total Return Bond index.
	To mitigate the impact of inflation, assets are allocated to inflation protection funds. These funds invest in indices that comprise the Dow Jones U.S. Select REIT, Dow Jones-UBS Commodity Total Return, S&P Global LargeMidCap Commodity and Resources, and Barclays Capital U.S. Treasury Inflation Protected Securities. Cash and short-term investments are held in a fund that purchases investment-grade short-term debt issued by governments and corporations.
	Responsibility for individual security selection rests with each investment manager, who is subject to guidelines specified by the EBC. These guidelines are designed to ensure consistency with overall plan objectives. The EBC sets performance objectives for each investment manager that are expected to be met over a three-year period or a complete market cycle, whichever is shorter. Performance and risk levels are regularly monitored to confirm policy compliance and that results are within expectations.
	Employee benefit plan regulations prohibit transactions between a fiduciary and parties-in-interest unless specifically provided for in ERISA. No restricted securities, such as letter stock or private placements, may be purchased for any investment fund. Questar securities may be considered for purchase at an investment manager's discretion, but within limitations prescribed by ERISA and other laws. There was no direct investment in Questar shares for the periods disclosed. Use of derivative securities by any investment manager is prohibited except where the EBC has given specific approval or where commingled funds are utilized that have previously adopted permitting guidelines.
	Fair value accounting standards establish a framework for defining, measuring and disclosing fair value in applying GAAP. The standards establish a fair value hierarchy with Levels 1, 2 and 3 ranging from the most observable to the least observable valuation inputs. Level 1 inputs are unadjusted quoted prices in active markets for identical assets or liabilities at the measurement date. Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. Level 3 inputs are unobservable inputs for the asset or liability. The following is a description of the valuation methodologies used at December 31, 2014 and 2013, to value qualified pension and other postretirement benefit plan assets. The Company's pension and other postretirement benefit plan assets did not include any investments measured using Level 3 inputs at December 31, 2014 or 2013.
	Corporate bonds and U.S. government securities: Corporate bonds and United States government corporation and agency securities are valued at the closing price reported on the markets on which the individual securities are traded, which in general are less active than the markets for common stocks and registered investment companies.
	Registered investment companies: Registered investment companies, also known as mutual funds, are valued at the closing price reported on the active markets on which the individual funds are traded.
	Commingled funds and 103-12 investment entity: These are investment vehicles generally restricted to institutional investors and are valued using the net asset value (NAV) of the fund. The NAV is based on the value of the underlying assets owned by the fund excluding transaction costs, and minus liabilities. The underlying assets are valued at the closing prices reported on the markets on which they are traded. No assets that were valued using a NAV methodology were subject to significant redemption restrictions or unfunded commitments on their valuation dates.
	The following tables set forth, by level within the fair value hierarchy, qualified pension and other postretirement benefit plan assets at fair value as of December 31, 2014 and 2013:
		December 31, 2014
		Level 1				Level 2				Total
		(in millions)
	Corporate bonds	$	—			$	1.5			$	1.5
	U.S. government securities	—				12.9				12.9
	Registered investment companies:
	Fixed income funds	114.9				—				114.9
	Inflation protection fund	34.5				—				34.5
	Domestic equity fund	1.6				—				1.6
	Commingled funds:
	Cash equivalent funds	—				6.7				6.7
	Domestic equity index fund	—				275.8				275.8
	Foreign equity growth fund	—				41.2				41.2
	Foreign equity index funds	—				77.8				77.8
	Corporate debt funds	—				91				91
	Inflation protection fund	—				33.1				33.1
	Foreign equity growth 103-12 investment entity	—				46.2				46.2
	Total	$	151			$	586.2			$	737.2
		December 31, 2013
		Level 1				Level 2				Total
		(in millions)
	Corporate bonds	$	—			$	1.7			$	1.7
	U.S. government securities	—				1.6				1.6
	Registered investment companies:
	Fixed income funds	110.7				—				110.7
	Inflation protection fund	29.4				—				29.4
	Domestic equity fund	2				—				2
	Commingled funds:
	Cash equivalent funds	—				3.5				3.5
	Domestic equity index fund	—				256.2				256.2
	Foreign equity growth fund	—				42.9				42.9
	Foreign equity index funds	—				76				76
Corporate debt funds	—				72.1				72.1
Inflation protection fund	—				28.8				28.8
Foreign equity growth 103-12 investment entity	—				47.1				47.1
Total	$	142.1			$	529.9			$	672
Pension plan benefits are based on the employee's age at retirement, years of service and highest average annual earnings during 72 consecutive semimonthly pay periods in the last 10 years of employment. Employees retiring on or after January 1, 2015 may elect either a lump-sum or an annuity benefit.
Postretirement health-care and life insurance benefits are provided only to employees hired before January 1, 1997. The Company pays a portion of the costs of health-care benefits determined by an employee's years of service and generally limited to 170% of the 1992 contribution for employees who retired after January 1, 1993. The Company amortized its transition obligation over a 20-year period that began in 1992 and ended in 2012.
The qualified pension projected benefit obligation was measured using the following assumptions at December 31:
	2014			2013
Discount rate	4.1	%		4.9	%
Rate of increase in compensation	5.5			5.5
Long-term return on assets	7			7.25
The nonqualified pension projected benefit obligation was measured using the following assumptions at December 31:
	2014			2013
Discount rate	2.9	%		3.3	%
Rate of increase in compensation	5.5			5.5
The postretirement accumulated benefit obligation was measured using the following assumptions at December 31:
	2014						2013
Discount rate	4				%		4.7				%
Long-term return on assets	7						7.25
Health-care inflation rate	7.5						8
	decreasing to						decreasing to
	4.5	%	by	2021			4.5	%	by	2021
Questar does not expect any plan assets to be returned during 2015. The qualified and nonqualified pension plan accumulated benefit obligation totaled $708.9 million at December 31, 2014. Plan obligations and fair value of all plan assets are shown in the following table:
	Pension								Other Postretirement Benefits
	Year Ended December 31,								Year Ended December 31,
	2014				2013				2014			2013
	(in millions)
Change in benefit obligation
Benefit obligation at beginning of year	$	702.7			$	760.3			$	86.3		$	94.4
Service cost	11.8				13.9				0.6			0.7
Interest cost	33.4				30.6				3.8			3.7
Change in plan assumptions	109.1				(73.6		)		4.9			(8.6		)
Actuarial (gain)	(0.7		)		(7.7		)		(3.0		)	(0.5		)
Benefits paid	(22.3		)		(20.8		)		(1.8		)	(3.4		)
Benefit obligation at end of year	834				702.7				90.8			86.3
Change in plan assets
Fair value of plan assets at beginning of year	628.2				510.8				43.8			37.6
Actual gain on plan assets	40.4				80.2				2.9			5.8
Company contributions to the plan	43.9				58				2.1			3.8
Benefits paid	(22.3		)		(20.8		)		(1.8		)	(3.4		)
Fair value of plan assets at end of year	690.2				628.2				47			43.8
Underfunded status (current and long-term)	$	(143.8	)		$	(74.5	)		$	(43.8	)	$	(42.5	)
The projected 2015 qualified pension plan funding is $44.0 million. Estimated benefit plan payments for the five years following 2014 and the subsequent five years aggregated are as follows:
	Pension				Other Postretirement Benefits
	Years Ending December 31,
	(in millions)
2015	$	67.3			$	4.3
2016	52.6				4.5
2017	47.2				4.6
2018	63				4.8
2019	53.6				5
2020 through 2024	265				26.3
The components of the net pension and other postretirement benefit costs are as follows. The net pension cost includes both the qualified and nonqualified pension plans.
	Pension											Other Postretirement Benefits
	Year Ended December 31,											Year Ended December 31,
	2014				2013				2012			2014				2013			2012
	(in millions)
Service cost	$	11.8			$	13.9			$	13.2		$	0.6			$	0.7		$	0.7
Interest cost	33.4				30.6				31.1			3.8				3.7			3.8
Expected return on plan assets	(43.6		)		(38.1		)		(31.8		)	(3.0		)		(2.6		)	(2.3		)
Prior service and other costs	0.6				1.1				1.1			—				—			1.6
Recognized net actuarial loss	15.4				28.9				24.9			0.7				3			2.5
Accretion of regulatory liability	—				—				—			0.8				0.5			0.3
Net periodic cost	$	17.6			$	36.4			$	38.5		$	2.9			$	5.3		$	6.6
Assumptions at January 1, used to calculate the qualified net pension cost for the years, were as follows:
	2014			2013			2012
Discount rate	4.9	%		4.2	%		4.8	%
Rate of increase in compensation	5.5			5.5			5.5
Long-term return on assets	7.25			7.25			7.25
Assumptions at January 1, used to calculate the nonqualified net pension cost for the years, were as follows:
	2014			2013			2012
Discount rate	3.3	%		2.4	%		3.3	%
Rate of increase in compensation	5.5			5.5			5.5
Assumptions at January 1, used to calculate the net postretirement benefit cost for the years, were as follows:
	2014						2013					2012
Discount rate	4.7				%		4				%	4.4				%
Long-term return on assets	7.25						7.25					7.25
Health-care inflation rate	8						8.5					7
	decreasing to						decreasing to					decreasing to
	4.5	%	by	2021			4.5	%	by	2021		5	%	by	2014
The 2015 estimated qualified and nonqualified net pension cost is $22.3 million. In 2015, $22.7 million of estimated net actuarial loss and $0.1 million of prior service cost for the pension plans will be amortized from AOCI. The 2015 estimated net postretirement benefit cost is $2.2 million, excluding accretion of a regulatory liability. In 2015, $1.3 million of estimated net actuarial loss for the postretirement benefit plans will be amortized from AOCI.
Postretirement benefit service and interest costs are sensitive to changes in the health-care inflation rate. A 1% increase in the health-care inflation rate would cause a minimal increase in the yearly service and interest costs and would increase the postretirement accumulated benefit obligation by $0.8 million. A 1% decrease in the health-care inflation rate would cause a minimal decrease in the yearly service and interest costs and would decrease the postretirement accumulated benefit obligation by $0.7 million.
Questar Gas and Questar Pipeline participate in Questar's qualified and nonqualified pension plans as well as its postretirement medical and life plans. Questar Gas's and Questar Pipeline's pension plan and postretirement medical and life insurance plan assets and benefit obligations cannot be separately determined because plan assets are not segregated or restricted to meet the companies' pension and postretirement medical and life obligations. If the companies were to withdraw from the plans, the pension and other postretirement obligations for Questar Gas and Questar Pipeline employees would be retained by the Questar plans. Pension and other postretirement benefit net cost and plan contribution information for Questar Gas and Questar Pipeline are shown below:
	Pension											Other Postretirement Benefits
	Year Ended December 31,											Year Ended December 31,
	2014				2013				2012			2014				2013			2012
	(in millions)
Questar Gas
Net periodic cost	$	8.5			$	18.1			$	19.7		$	0.8			$	2.4		$	3.3
Share of total plan contributions	21.8				29.6				39.4			1.1				2			1.9
Questar Pipeline
Net periodic cost	$	3.2			$	6.7			$	7.4		$	(0.4	)		$	0.2		$	1.2
Share of total plan contributions	8.1				11.2				15.1			0.4				0.7			0.6
401(k) Retirement Income Plan
The 401(k) Retirement Income Plan (401(k) Plan), formerly known as the Employee Investment Plan, is a defined contribution pension plan that allows eligible employees to purchase shares of Questar common stock or other investments through payroll deduction at the fair market value on the transaction date. The Company contributes an overall match of 100% of employees' purchases up to a maximum of 6% of their qualifying earnings. Starting in January 2015, qualified employees who are not eligible for the defined benefit pension plans receive an additional non-matching employer contribution to their 401(k) Plan accounts equal to 4% of their qualifying prior year earnings. To satisfy employee purchases of Questar stock, the 401(k) Plan trustee may purchase Questar shares on the open market with cash received or Questar may issue new shares.
Questar, Questar Gas and Questar Pipeline recognize expense equal to 401(k) Plan employer matching and non-matching contributions earned by employees during the year. Questar's expense amounted to $9.1 million in 2014, $7.2 million in 2013, and $7.3 million in 2012. Questar Gas's 401(k) Plan expense was $4.1 million in 2014, $3.4 million in 2013 and $3.6 million in 2012. Questar Pipeline's 401(k) Plan expense was $1.8 million in 2014 and $1.4 million in 2013 and 2012.

Operations_by_Line_of_Business

Operations by Line of Business (Notes)	12 Months Ended
	Dec. 31, 2014
Segment Reporting [Abstract]
Operations by Line of Business	Note 14 - Operations by Line of Business
	Questar's three principal complementary lines of business include Questar Gas, which provides retail natural gas distribution in Utah, Wyoming and Idaho; Wexpro, which develops and produces natural gas for Questar Gas customers; and Questar Pipeline, which operates interstate natural gas pipelines and storage facilities and provides other energy services. Line-of-business information is presented according to senior management's basis for evaluating performance and considering differences in the nature of products, services and regulation, among other factors.
	The following is a summary of operations by line of business for the three years ended December 31, 2014:
		Questar Consol.			Interco. Trans.			Questar Gas			Wexpro			Questar Pipeline			Corp and Other
		(in millions)
	2014
	Revenues
	From unaffiliated customers	$	1,189.30		$	—		$	960.9		$	35.6		$	190.2		$	2.6
	From affiliated companies	—			(424.0		)	—			350.3			73.7			—
	Total Revenues	1,189.30			(424.0		)	960.9			385.9			263.9			2.6
	Operating Expenses
	Cost of sales	186.3			(423.4		)	604.8			—			4			0.9
	Operating and maintenance	189			—			122.5			31.5			34.1			0.9
	General and administrative	127.9			(0.6		)	52.8			32.6			44.1			(1.0		)
	Production and other taxes	66.2			—			17.8			37.6			9.1			1.7
	Depreciation, depletion and amortization	213.7			—			53.6			100.5			54.5			5.1
	Abandonment and impairment	2			—			—			2			—			—
	Total Operating Expenses	785.1			(424.0		)	851.5			204.2			145.8			7.6
	Net gain (loss) from asset sales	1.2			—			0.1			1.6			(0.5		)	—
	Operating Income (Loss)	405.4			—			109.5			183.3			117.6			(5.0		)
	Interest and other income (expense)	6.6			(0.4		)	5.9			1.1			1.2			(1.2		)
	Income from unconsolidated affiliate	3.5			—			—			—			3.5			—
	Interest expense	(63.1		)	0.4			(28.2		)	(0.1		)	(26.1		)	(9.1		)
	Income taxes	(125.9		)	—			(32.0		)	(61.5		)	(35.6		)	3.2
	Net Income (Loss)	$	226.5		$	—		$	55.2		$	122.8		$	60.6		$	(12.1	)
	Identifiable assets	$	4,249.70		$	—		$	1,911.10		$	977.5		$	1,232.40		$	128.7
	Goodwill	9.8			—			5.6			—			4.2			—
	Investment in unconsolidated affiliate	24.7			—			—			—			24.7			—
	Cash capital expenditures, including acquisitions	371.5			—			174.7			114.4			59.4			23
	Accrued capital expenditures, including acquisitions	359.7			—			161.5			114.7			61.3			22.2
	2013
	Revenues
	From unaffiliated customers	$	1,220.00		$	—		$	985.2		$	45.1		$	189.5		$	0.2
	From affiliated companies	—			(372.1		)	0.6			294.8			76.7			—
	Total Revenues	1,220.00			(372.1		)	985.8			339.9			266.2			0.2
	Operating Expenses
	Cost of sales	285.9			(370.9		)	650.6			—			6.1			0.1
	Operating and maintenance	174.3			—			113.1			28.6			32.5			0.1
	General and administrative	121			(0.6		)	52.5			28.7			46.7			(6.3		)
	Production and other taxes	57.4			—			18			28.3			9.3			1.8
	Depreciation, depletion and amortization	194.8			—			49.7			85.8			55.5			3.8
	Asset impairment	80.6			—			—			—			80.6			—
	Other operating expenses	—			(0.6		)	—			0.6			—			—
	Total Operating Expenses	914			(372.1		)	883.9			172			230.7			(0.5		)
	Net loss from asset sales	(0.2		)	—			—			(0.2		)	—			—
	Operating Income	305.8			—			101.9			167.7			35.5			0.7
	Interest and other income (expense)	9.9			(0.7		)	5.1			5			1.8			(1.3		)
	Income from unconsolidated affiliate	3.7			—			—			—			3.7			—
	Interest expense	(56.9		)	0.7			(22.3		)	(0.1		)	(25.8		)	(9.4		)
	Income taxes	(101.3		)	—			(31.9		)	(62.0		)	(7.0		)	(0.4		)
	Net Income (Loss)	$	161.2		$	—		$	52.8		$	110.6		$	8.2		$	(10.4	)
	Identifiable assets	$	4,054.30		$	—		$	1,762.00		$	993.8		$	1,222.80		$	75.7
	Goodwill	9.8			—			5.6			—			4.2			—
	Investment in unconsolidated affiliate	25.6			—			—			—			25.6			—
	Cash capital expenditures, including acquisitions	503.7			—			166.2			249.5			73.4			14.6
	Accrued capital expenditures, including acquisitions	505.6			—			177.3			240.7			70			17.6
	2012
	Revenues
	From unaffiliated customers	$	1,098.90		$	—		$	859.7		$	36.1		$	203.1		$	—
	From affiliated companies	—			(351.0		)	2.5			274.1			74.4			—
	Total Revenues	1,098.90			(351.0		)	862.2			310.2			277.5			—
	Operating Expenses
	Cost of sales	192.3			(347.7		)	533.3			—			6.7			—
	Operating and maintenance	180.8			(0.2		)	119			26.8			35.2			—
General and administrative	120.8			(0.6		)	51.2			26.8			50			(6.6		)
Retirement incentive	4.9			—			2.4			0.2			0.9			1.4
Production and other taxes	47.9			—			16.2			20.8			9.1			1.8
Depreciation, depletion and amortization	181.6			—			47.2			77.4			54.3			2.7
Other operating expenses	—			(2.5		)	—			2.5			—			—
Total Operating Expenses	728.3			(351.0		)	769.3			154.5			156.2			(0.7		)
Net gain from asset sales	5.1			—			—			2.4			2.7			—
Operating Income	375.7			—			92.9			158.1			124			0.7
Interest and other income (expense)	7			(0.7		)	5.5			2.8			0.6			(1.2		)
Income from unconsolidated affiliate	3.7			—			—			—			3.7			—
Interest expense	(57.9		)	0.7			(24.1		)	—			(26.3		)	(8.2		)
Income taxes	(116.5		)	—			(27.2		)	(57.0		)	(37.3		)	5
Net Income (Loss)	$	212		$	—		$	47.1		$	103.9		$	64.7		$	(3.7	)
Identifiable assets	$	3,803.30		$	—		$	1,593.50		$	836.1		$	1,298.90		$	74.8
Goodwill	9.8			—			5.6			—			4.2			—
Investment in unconsolidated affiliate	26.5			—			—			—			26.5			—
Cash capital expenditures	370.7			—			162.1			144.5			60.6			3.5
Accrued capital expenditures	366.3			—			161.2			145.5			58.6			1

RelatedParty_Transaction_Notes

Related-Party Transaction (Notes)	12 Months Ended
	Dec. 31, 2014
Related Party Transactions [Abstract]
Related-Party Transactions	Note 15 - Related-Party Transactions
	Questar Gas
	In 2014 and 2013 Questar Gas provided technical services to affiliates. In 2012, Questar Gas also provided communication services to affiliates. Questar Gas provided these services at its cost and charged $6.1 million in 2014, $6.7 million in 2013 and $13.6 million in 2012. The majority of these costs are allocated. The allocation methods are based on the specific nature of the charges. Management believes that the allocation methods are reasonable.
	Questar Gas has reserved transportation capacity on Questar Pipeline's system for 916 Mdth per day during the heating season and 841 Mdth per day during off-peak months. Questar Gas periodically releases excess capacity and receives a credit from Questar Pipeline for the released capacity revenues and a portion of Questar Pipeline's interruptible transportation revenues. Questar Gas paid for transportation, storage and processing services provided by Questar Pipeline and a subsidiary amounting to $72.9 million in 2014, $73.0 million in 2013 and $73.6 million in 2012, which included demand charges. The costs of these services were included in the cost of natural gas sold.
	Under the terms of the Wexpro agreements, Questar Gas receives a portion of Wexpro's income from oil and NGL operations after recovery of Wexpro's operating expenses and a return on investment. This amount, which is included in revenues and reduces amounts billed to gas distribution customers, was $0.6 million in 2013 and $2.5 million in 2012. There was no such revenue in 2014. The amounts that Questar Gas paid Wexpro for the operation of cost-of-service gas properties were $349.7 million in 2014, $294.6 million in 2013 and $274.0 million in 2012. Questar Gas reports these amounts in the cost of natural gas sold.
	Questar Gas had a lease with an affiliate for space in an office building located in Salt Lake City, Utah, which expired on April 30, 2012. Rent expense was $0.4 million in 2012.
	Questar charged Questar Gas for certain administrative functions amounting to $47.8 million in 2014, $48.4 million in 2013 and $49.3 million in 2012. These costs are included in operating expenses and are generally allocated based on each affiliated company's proportional share of revenues less product costs; property, plant and equipment; and labor costs. Management believes that the allocation method is reasonable.
	Questar Pipeline charged Questar Gas for communication services amounting to $3.7 million in 2014 and 2013. These costs are included in operating expenses and are allocated based on usage.
	Questar Gas borrowed cash from Questar and incurred interest expense of $0.1 million in 2014, $0.5 million in 2013 and $0.6 million in 2012.
	Questar Pipeline
	Questar Pipeline receives a substantial portion of its revenues from Questar Gas. Total revenues received from Questar Gas were $73.3 million in 2014, $76.3 million in 2013 and $74.0 million in 2012.
	Beginning in 2013, Questar Pipeline provided communication services to affiliates. Questar Pipeline provided these services at its cost and charged $5.0 million in 2014 and $4.9 million in 2013. The majority of these costs are allocated based on usage.
	In 2014 and 2013 Questar Gas provided technical services to Questar Pipeline. In 2012, Questar Gas also provided communication services to Questar Pipeline. Questar Gas provided these services at its cost of $3.8 million in 2014, $3.5 million in 2013 and $8.8 million in 2012. The majority of these costs are allocated and included in operating expenses. The allocation methods are based on the specific nature of the charges. Management believes that the allocation methods are reasonable.
	Questar charged Questar Pipeline for certain administrative functions amounting to $25.8 million in 2014, $28.2 million in 2013 and $24.5 million in 2012. These costs are included in operating expenses and are generally allocated based on each affiliate's proportional share of revenues less product costs; property, plant and equipment; and labor costs. Management believes that the allocation method is reasonable.
	Questar Pipeline had a lease with an affiliate for space in an office building located in Salt Lake City, Utah, which expired on April 30, 2012. Rent expense was $0.3 million in 2012.
	Questar Pipeline loaned excess funds to Questar and earned interest income of $0.1 million in 2014 and 2012 and $0.2 million in 2013.

Asset_Impairments_Notes

Asset Impairments (Notes)	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment Impairment or Disposal [Abstract]
Asset Impairments	Note 16 - Asset Impairments
	During the second quarter of 2014, Wexpro recorded a pre-tax abandonment and impairment charge of $2.0 million for its share of the remaining investment in the Brady field. Wexpro concluded that the field had reached the end of its productive life because it was no longer economical to produce natural gas and oil.
	During the third quarter of 2013, Questar Pipeline updated its five-year forecast for the eastern segment of Southern Trails Pipeline, which resulted in revised projections of higher operating expenses, including right-of-way and pipeline safety costs. Current and projected market rates for natural gas transportation between the San Juan Basin and California markets did not cover these increasing operating expenses. Because of changes in expected cash flows in the third quarter of 2013 and the lack of progress in selling or recontracting this pipeline, Questar Pipeline recorded a noncash impairment of its entire investment in the eastern segment of Southern Trails Pipeline of $80.6 million, or $52.4 million after income taxes. Questar Pipeline used a probability-weighted discounted cash flow analysis that included significant inputs such as Questar Pipeline's cost of capital and assumptions regarding future transportation rates and operating costs.

Wexpro_Acquisitions_of_Produci

Wexpro Acquisitions of Producing Properties (Notes)	12 Months Ended
	Dec. 31, 2014
Wexpro Acquisition of Producing Properties [Abstract]
Wexpro Acquisition of Producing Properties	Note 17 - Wexpro Acquisitions of Producing Properties
	On December 17, 2014, Wexpro acquired an additional interest in natural gas-producing properties in existing Wexpro-operated assets in the Canyon Creek Unit of southwestern Wyoming's Vermillion Basin for about $52.4 million. This is a "bolt-on" acquisition to the company's current Canyon Creek assets, which are governed by the 1981 Wexpro Agreement. Under the terms of the Wexpro II Agreement, this property must be submitted to the Commissions to be considered for cost-of-service treatment for the benefit of Questar Gas customers.
	Because it contains producing properties, the Canyon Creek acquisition is accounted for under the accounting guidance for business combinations. Summary information related to the Canyon Creek acquisition is shown in the table below. Amounts are provisional and are subject to post-closing adjustments.
		(in millions)
	Fair value of consideration transferred:
	Net cash paid	$	52.4
	Recognized amounts of identifiable assets acquired and liabilities assumed, at fair value:
	Proved properties	$	54.1
	Asset retirement obligations	(1.7		)
	Total fair value of identifiable net assets acquired	$	52.4
	Canyon Creek acquisition-related transaction costs of $0.3 million for 2014 are included in general and administrative expense on Questar's Consolidated Statement of Income. Amounts included in Questar's Consolidated Statements of Income for the period between the December 17, 2014 closing date and December 31, 2014 related to this acquisition are de minimis. Supplemental pro forma income information related to the Canyon Creek acquisition is not presented because it is immaterial to Questar's consolidated revenues, expenses and income in all periods presented.
	On September 4, 2013, Wexpro completed the transaction announced in July 2013 to acquire an additional interest in natural gas-producing properties in the Trail Unit of southwestern Wyoming's Vermillion Basin (Trail acquisition) for $104.3 million, after post-closing adjustments. This acquisition was an addition to the company’s existing Trail assets, which are governed by the 1981 Wexpro Agreement. In the first quarter of 2014, the Public Service Commission of Utah and the Wyoming Public Service Commission (the Commissions) approved the inclusion of these properties in the Wexpro II Agreement, effective February 1, 2014.

Quarterly_Financial_Informatio

Quarterly Financial Information - Unaudited (Notes)	12 Months Ended
	Dec. 31, 2014
Selected Quarterly Financial Information [Abstract]
Quarterly Financial Information (Unaudited)	Note 18 - Quarterly Financial Information (Unaudited)
	Following is a summary of unaudited quarterly financial information:
		First Quarter		Second Quarter		Third Quarter			Fourth Quarter		Year
		(in millions, except per-share amounts)
	2014
	Revenues	$	456.9	$	201.3	$	157.9		$	373.2	$	1,189.30
	Operating income	148.3		76.5		68.9			111.7		405.4
	Net income	85.1		40.3		38.6			62.5		226.5
	Basic earnings per common share	$	0.48	$	0.23	$	0.22		$	0.36	$	1.29
	Diluted earnings per common share	0.48		0.23		0.22			0.35		1.29
	2013
	Revenues	$	476.9	$	195.6	$	149.6		$	397.9	$	1,220.00
	Operating income (loss)	131.4		71.2		(19.4		)	122.6		305.8
	Net income (loss)	72.9		39.4		(19.2		)	68.1		161.2
	Asset impairment charge, before income taxes	—		—		80.6			—		80.6
	Basic earnings (loss) per common share	$	0.42	$	0.22	$	(0.11	)	$	0.39	$	0.92
	Diluted earnings (loss) per common share	0.41		0.22		(0.11		)	0.39		0.92

Supplemental_Gas_and_Oil_Infor

Supplemental Gas and Oil Information - Unaudited (Notes)	12 Months Ended
	Dec. 31, 2014
Extractive Industries [Abstract]
Supplemental Gas and Oil Information (Unaudited)	Note 19 - Supplemental Gas and Oil Information (Unaudited)
	The Company is making the following supplemental disclosures of gas and oil producing activities, in accordance with accounting standards for extractive activities - oil and gas and SEC Regulation S-X.
	The Company uses the successful efforts accounting method for its gas and oil properties.
	The substantial majority of the following information relates to cost-of-service gas and oil properties managed and developed by Wexpro and governed by the Wexpro agreements.
	In December 2014, Wexpro completed the Canyon Creek acquisition. Properties acquired in this acquisition were not included in a cost-of-service arrangement as of December 31, 2014. In September 2013, Wexpro completed the Trail acquisition. In the first quarter of 2014, the PSCU and PSCW approved the inclusion of the Trail acquisition properties in the Wexpro II Agreement. The 2014 and 2013 supplemental gas and oil information includes these acquisitions, as applicable. See Note 17 for additional information on these acquisitions.
	Capitalized Costs
	Capitalized costs of gas and oil properties and the related amounts of accumulated depreciation, depletion and amortization are shown below:
		December 31,
		2014				2013
		(in millions)
	Wexpro
	Proved properties	$	1,675.60			$	1,625.50
	Unproved properties	12.8				0.9
	Total Wexpro capitalized costs	1,688.40				1,626.40
	Accumulated depreciation, depletion and amortization	(757.3		)		(689.8		)
	Net Wexpro capitalized costs	931.1				936.6
	Net Questar Gas capitalized costs	6.4				7.2
	Net capitalized costs	$	937.5			$	943.8
	Costs Incurred
	The costs incurred for gas and oil development activities are displayed in the table below. The costs incurred to develop proved undeveloped reserves were $28.9 million in 2014, $106.3 million in 2013 and $50.4 million in 2012.
		Year Ended December 31,
		2014				2013				2012
		(in millions)
	Property acquisition
	Unproved	$	11.9			$	0.3			$	0.6
	Proved	54.1				106.4				—
	Exploration (capitalized and expensed)	1.6				—				—
	Development	49.1				133.1				146.9
	Total costs incurred	$	116.7			$	239.8			$	147.5
	Results of Operations
	Following are the results of operations for gas- and oil-producing activities, excluding corporate overhead and interest costs:
		Year Ended December 31,
		2014				2013				2012
		(in millions)
	Revenues
	From unaffiliated customers	$	35.6			$	45.1			$	36.1
	From affiliated company(1)	350.3				294.8				274.1
	Total revenues	385.9				339.9				310.2
	Production costs	67.5				57.5				50.1
	Exploration expenses	1.6				—				—
	Depreciation, depletion and amortization	100.5				85.8				77.4
	Abandonment and impairment	2				—				—
	Total expenses	171.6				143.3				127.5
	Revenues less expenses	214.3				196.6				182.7
	Income taxes	(71.4		)		(70.5		)		(64.8		)
	Results of operations for gas- and oil-producing activities (excluding corporate overhead and interest costs)	$	142.9			$	126.1			$	117.9
	(1) Primarily represents revenues received from Questar Gas pursuant to the Wexpro agreements. Revenues include reimbursement of general and administrative expenses amounting to $30.5 million in 2014, $27.5 million in 2013 and $26.5 million in 2012.
	Estimated Quantities of Proved Gas and Oil Reserves
	Estimates of proved gas and oil reserves have been prepared in accordance with professional engineering standards and the Company's established internal controls. The estimates were prepared by Wexpro's reservoir engineers, individuals who possess professional qualifications and demonstrated competency in reserves estimation and evaluation. SEC guidelines with respect to standard economic assumptions are not applicable to the large proportion of Wexpro gas reserves that are managed, developed, produced and delivered to Questar Gas at cost of service. The SEC acknowledges this potential circumstance and provides that companies may give appropriate recognition to differences arising because of the effect of the rate-making process. Accordingly, in cases where differences arise because of the effect of the rate-making process, Wexpro uses a minimum-producing rate or maximum well-life limit to determine the ultimate quantity of reserves attributable to each well.
	The Company annually reviews all proved undeveloped reserves to ensure an appropriate plan for development exists. All proved undeveloped reserves are converted to proved developed reserves within five years of the proved undeveloped reserve booking. At December, 2014, all of the Company's proved undeveloped reserves were scheduled to be developed within five years from the date such locations were initially disclosed as proved undeveloped reserves. Wexpro converted 7% of prior year-end proved undeveloped reserves to developed status in 2014, 42% in 2013 and 15% in 2012.
	Revisions of prior estimates reflect the addition of new proved undeveloped reserves associated with current five-year development plans, revisions to prior proved undeveloped reserves, revisions to infill drilling development plans, as well as the transfer of proved undeveloped reserves to unproved reserve categories due to changes in development plans. The negative revisions reflected in the 2013 reserve estimates are due in part to an increase in well spacing in the Pinedale field based on 2013 drilling results. The negative revisions in 2014 are primarily due to the impact on proved undeveloped reserves from significant changes in the Company's five-year development plans based on the drop in natural gas and oil prices at the end of 2014.
	In establishing reserves, the SEC allows the use of techniques that have been field tested and demonstrated to provide reasonably certain results with consistency and repeatability in the formation being evaluated or in an analogous formation. In general, the Company uses numerous data elements and analysis techniques in the estimation of proved reserves. These data elements and techniques include, but are not limited to, production tests, well performance data, decline curve analysis, wireline logs, core data, pressure transient analysis, seismic data and interpretation, and material balance calculations. The Company utilizes these reliable technologies to book proved reserves, however, no reserves were recorded from increasing recovery factor estimates or from extending down-dip reservoir limits associated with the use of reliable technology.
	Wexpro's estimates of proved reserves were made by the Company's engineers and are the responsibility of management. The Company requires that reserve estimates be made by qualified reserves estimators (QREs), as defined by the Society of Petroleum Engineers' standards. The QREs interact with engineering, land, and geoscience personnel to obtain the necessary data for projecting future production, costs, net revenues and ultimate recoverable reserves. Management approves the QREs' reserve estimates annually. All QREs receive ongoing education on the fundamentals of SEC reserves reporting through internal and external training over the policies for estimating and recording reserves in compliance with applicable SEC definitions and guidance.
	Estimated quantities of proved gas and oil reserves are set forth below:
		Natural Gas			Oil and NGL			Natural Gas Equivalents
		(Bcf)			(Mbbl)			(Bcfe)
	Proved Reserves
	Balances at December 31, 2011	758.7			5,344			790.7
	Revisions of previous estimates	(158.4	)		(122	)		(159.0	)
	Extensions and discoveries	154.4			1,612			164
	Production	(57.5	)		(665	)		(61.5	)
	Balances at December 31, 2012	697.2			6,169			734.2
	Revisions of previous estimates	(112.8	)		(1,348	)		(120.8	)
	Extensions and discoveries	153.5			857			158.6
Purchase of reserves in place	133.9			556			137.2
Production	(60.6	)		(617	)		(64.3	)
Balances at December 31, 2013	811.2			5,617			844.9
Revisions of previous estimates	(220.2	)		(442	)		(222.7	)
Extensions and discoveries	4			205			5.2
Purchase of reserves in place	35.9			157			36.8
Sale of reserves in place	(0.5	)		(219	)		(1.9	)
Production	(64.3	)		(587	)		(67.8	)
Balances at December 31, 2014	566.1			4,731			594.5
Proved Developed Reserves
Balances at December 31, 2011	491.2			4,420			517.7
Balances at December 31, 2012	523.9			4,967			553.7
Balances at December 31, 2013	560			4,384			586.3
Balances at December 31, 2014	552.9			4,678			581
Proved Undeveloped Reserves
Balances at December 31, 2011	267.5			924			273
Balances at December 31, 2012	173.3			1,202			180.5
Balances at December 31, 2013	251.2			1,233			258.6
Balances at December 31, 2014	13.2			53			13.5
Standardized Measure of Future Net Cash Flows Relating to Non-Cost-of-Service Proved Reserves
The above December 31, 2014 balance of total proved reserves includes 36.6 Bcfe of non-cost-of-service reserves associated with the December 2014 Canyon Creek acquisition. These reserves were not covered under any cost-of-service arrangement as of the filing of this Annual Report. In accordance with the Wexpro II Agreement, these properties must be submitted to the Commissions to be considered for Wexpro II treatment.
The standardized measure of future net cash flows applies only to non-cost-of-service reserves. Information on the standardized measure of future net cash flows has not been included for cost-of-service activities because the operations of and return on investment for such properties are regulated by the Wexpro agreements.
Future net cash flows were calculated at December 31, 2014 by applying prices used in estimating 2014 reserves. Year-end production costs, development costs and appropriate statutory income tax rates, with consideration of future tax rates already legislated, were used to compute the future net cash flows. All cash flows were discounted at 10% to reflect the time value of cash flows, without regard to the risk of specific properties.
The assumptions used to derive the standardized measure of future net cash flows, including the 10% discount rate, are those required by accounting standards and do not necessarily reflect the Company's expectations. The usefulness of the standardized measure of future net cash flows is impaired because of the reliance on reserve estimates and production schedules that are inherently imprecise. Furthermore, information contained in the following disclosure may not represent realistic assessments of future cash flows, nor should the standardized measure of future net cash flows be viewed as representative of the current value of the Company's reserves.
Management considers a number of factors when making investment and operating decisions. These include estimates of probable and proved reserves and varying price and cost assumptions considered more representative of a range of anticipated economic conditions. The standardized measure of future net cash flows relating to non-cost-of-service proved reserves is presented in the table below:
	December 31,
	2014
	(in millions)
Future cash inflows	$	190.7
Future production costs	(65.9		)
Future income tax expenses	(43.7		)
Future net cash flows	81.1
10% annual discount for estimated timing of net cash flows	(34.4		)
Standardized measure of discounted future net cash flows	$	46.7
The principal sources of change in the standardized measure of future net cash flows relating to non-cost-of-service proved reserves are presented in the table below:
	Year Ended December 31,
	2014
	(in millions)
Balance at beginning of year	$	—
Net increase due to purchases of reserves in place	46.8
Sales of gas and oil produced during the period, net of production costs	(1.5		)
Net change in prices and production costs related to future production	0.8
Accretion of discount	0.6
Net Change	46.7
Balance at end of year	$	46.7

Schedule_of_Valuation_and_Qual

Schedule of Valuation and Qualifying Accounts (Notes)	12 Months Ended
	Dec. 31, 2014
Valuation and Qualifying Accounts [Abstract]
Schedule of Valuation and Qualifying Accounts	Financial Statement Schedules:
	QUESTAR CORPORATION
	Schedule of Valuation and Qualifying Accounts
	Column A	Column B		Column C Amounts charged		Column D Deductions for accounts written off and other			Column E
	Description	Beginning Balance		to expense					Ending Balance
		(in millions)
	Year Ended December 31, 2014
	Allowance for bad debts	$	1.7	$	1.7	$	(1.7	)	$	1.7
	Year Ended December 31, 2013
	Allowance for bad debts	3.1		0.2		(1.6		)	1.7
	Year Ended December 31, 2012
	Allowance for bad debts	3.2		1.2		(1.3		)	3.1
	QUESTAR GAS COMPANY
	Schedule of Valuation and Qualifying Accounts
	Column A	Column B		Column C Amounts charged		Column D Deductions for accounts written off and other			Column E
	Description	Beginning Balance		to expense					Ending Balance
		(in millions)
	Year Ended December 31, 2014
	Allowance for bad debts	$	1.4	$	1.7	$	(1.7	)	$	1.4
	Year Ended December 31, 2013
	Allowance for bad debts	2.8		0.2		(1.6		)	1.4
	Year Ended December 31, 2012
	Allowance for bad debts	2.9		1.1		(1.2		)	2.8
	QUESTAR PIPELINE COMPANY
	Schedule of Valuation and Qualifying Accounts
	Column A	Column B		Column C Amounts charged		Column D Deductions for accounts written off and other			Column E
	Description	Beginning Balance		to expense					Ending Balance
		(in millions)
	Year Ended December 31, 2014
	Allowance for bad debts	$	0.3	$	—	$	—		$	0.3
	Year Ended December 31, 2013
	Allowance for bad debts	0.3		—		—			0.3
	Year Ended December 31, 2012
	Allowance for bad debts	0.3		0.1		(0.1		)	0.3

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Line Items]
Nature of Business	A. Nature of Business
	Questar Corporation (Questar or the Company) is a Rockies-based integrated natural gas company with three principal complementary and wholly-owned lines of business:
	•	Questar Gas Company (Questar Gas) provides retail natural gas distribution in Utah, Wyoming and Idaho.
	•	Wexpro Company (Wexpro) develops and produces natural gas from cost-of-service reserves for Questar Gas customers.
	•	Questar Pipeline Company (Questar Pipeline) operates interstate natural gas pipelines and storage facilities in the western United States and provides other energy services.
	Questar is headquartered in Salt Lake City, Utah. Shares of Questar common stock trade on the New York Stock Exchange (NYSE:STR).
Principles of Consolidation	B. Principles of Consolidation
	The Questar and Questar Pipeline consolidated financial statements contain the accounts of the parent companies and their majority-owned or controlled subsidiaries. The consolidated financial statements were prepared in accordance with U.S. generally accepted accounting principles (GAAP) and with the instructions for Annual Reports on Form 10-K and SEC Regulations S-X and S-K. All significant intercompany accounts and transactions have been eliminated in consolidation.
Investment in Unconsolidated Affiliate	C. Investment in Unconsolidated Affiliate
	Questar and Questar Pipeline use the equity method to account for an investment in an unconsolidated affiliate where they do not have control, but have significant influence. The investment in the unconsolidated affiliate on the Consolidated Balance Sheets equals Questar Pipeline's proportionate share of equity reported by the unconsolidated affiliate. The investment is assessed for possible impairment when events indicate that the fair value of the investment may be below Questar Pipeline's carrying value. When such a condition is deemed to be other-than-temporary, the carrying value of the investment is written down to its fair value, and the amount of the write-down is included in the determination of net income.
	White River Hub, LLC, a limited liability company and FERC-regulated transporter of natural gas, is the sole unconsolidated affiliate. Questar Pipeline owns 50% of White River Hub, LLC, and is the operator.
Use of Estimates	D. Use of Estimates
	The preparation of financial statements and notes in conformity with GAAP requires that management formulate estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses and the disclosure of contingent assets and liabilities. The Company also incorporates estimates of proved developed and total proved gas and oil reserves in the calculation of depreciation, depletion and amortization rates of its gas and oil properties. Changes in estimated quantities of the Company's reserves could impact its reported financial results as well as disclosures regarding the quantities of proved gas and oil reserves. Actual results could differ from these estimates.
Revenue Recognition	E. Revenue Recognition
	Questar Gas
	Questar Gas records revenues in the period that gas is delivered, including gas delivered to residential and commercial customers but not billed as of the end of the accounting period. Unbilled gas deliveries are estimated for the period from the date meters are read to the end of the month. Approximately one-half month of revenue is estimated in any period. Gas costs and other variable costs are recorded on the same basis to ensure proper matching of revenues and expenses. Questar Gas's tariff allows for monthly adjustments to customer bills to approximate the effect of abnormal weather on non-gas revenues. The weather-normalization adjustment significantly reduces the impact of weather on gas-distribution earnings. The PSCU and PSCW approved a conservation enabling tariff (CET) to promote energy conservation. Under the CET, Questar Gas non-gas revenues are decoupled from the volume of gas used by customers. The tariff specifies an allowed monthly revenue per customer, with differences to be deferred and recovered from or refunded to customers through periodic rate adjustments. Rate adjustments occur every six months under the CET. The adjustments amortize deferred CET amounts over a 12-month period. These adjustments are limited to 5% of non-gas revenues.
	Questar Gas allows customers the option of paying an estimated fixed monthly bill throughout the year on a budget-billing program. The estimated payments are adjusted to actual usage annually. Amounts collected from customers under this program in excess of gas deliveries are recorded on the Consolidated Balance Sheets as customer advances. The budget-billing option does not impact revenue recognition. Questar Gas may collect revenues subject to possible refunds and establish reserves pending final orders from regulatory agencies.
	Wexpro
	Wexpro recognizes revenues in the period that services are provided or products are delivered. In accordance with the Wexpro agreements, production from the gas properties operated by Wexpro is delivered to Questar Gas at Wexpro's cost of providing this service, including an after-tax return on Wexpro's investment. Wexpro sells crude oil and NGL production from certain producing properties at market prices, with the revenues used to recover operating expenses and to provide Wexpro a return on its investment. Any operating income remaining after recovery of expenses and Wexpro's return on investment is divided between Questar Gas and Wexpro, with Wexpro retaining 46%. Amounts received by Questar Gas from the sharing of Wexpro's oil and NGL income are used to reduce natural gas costs to utility customers.
	Wexpro's investment base consists of its costs of acquired properties and commercial wells and related facilities, and is adjusted for working capital and reduced for deferred income taxes and accumulated depreciation, depletion and amortization. Property acquisition costs only pertain to properties that have been approved under the Wexpro II Agreement. Wexpro may collect revenues subject to possible refunds and establish reserves pending final calculation of the after-tax return on investment, which is adjusted annually.
	Revenues associated with the sale of gas, oil and NGL are accounted for using the sales method, whereby revenue is recognized as gas, oil and NGL are sold to purchasers. A liability is recorded to the extent that Wexpro has sold or delivered volumes in excess of its share of remaining gas and oil reserves in the underlying properties.
	Questar Pipeline
	Questar Pipeline and subsidiaries recognize revenues in the period that services are provided or products are delivered. The straight-fixed-variable rate design used by Questar Pipeline, which allows for recovery of substantially all fixed costs in the demand or reservation charge, reduces the earnings impact of volume changes on gas transportation and storage operations. Questar Pipeline may collect revenues subject to possible refunds and establish reserves pending final orders from regulatory agencies.
Cost of Sales	F. Cost of Sales
	Questar Gas obtains the majority of its gas supply from Wexpro's cost-of-service production and pays Wexpro an operator service fee based on the terms of the Wexpro agreements. Questar Gas also obtains transportation and storage services from Questar Pipeline. These intercompany revenues and expenses are eliminated in the Questar Consolidated Statements of Income by reducing revenues and cost of sales. The underlying costs of Wexpro's production and Questar Pipeline's transportation and storage services are disclosed in other categories in the Consolidated Statements of Income, including operating and maintenance expense and depreciation, depletion and amortization expense. During the second and third quarters of the year, a significant portion of the natural gas from Wexpro production is injected into underground storage. This gas is withdrawn from storage as needed during the heating season in the first and fourth quarters. The cost of natural gas sold is credited with the value of natural gas as it is injected into storage and debited as it is withdrawn from storage. The reported balance in consolidated cost of sales may be a negative amount during the second and third quarters because of the entries to record injection of gas into storage and the elimination of intercompany transactions.
Regulation	G. Regulation
	The Company applies the regulatory accounting principles to its rate-regulated businesses. Under these principles, the Company records regulatory assets and liabilities that would not be otherwise recorded under GAAP for non-rate-regulated entities. Regulatory assets and liabilities record probable future revenues or expenses associated with certain charges or credits that will be recovered from or refunded to customers through the rate-making process.
	Questar Gas accounts for purchased-gas costs in accordance with procedures authorized by the PSCU and the PSCW. Purchased-gas costs that are different from those provided for in present rates are accumulated and recovered or credited through future rate changes. Questar Gas may hedge a portion of its natural gas supply to mitigate price fluctuations for gas-distribution customers. The regulatory commissions allow Questar Gas to record periodic mark-to-market adjustments for commodity-price derivatives in the purchased-gas adjustment account. Questar did not have any commodity-price derivatives at December 31, 2014 or 2013. See Note 11 for a description and comparison of regulatory assets and liabilities as of December 31, 2014 and 2013.
	Wexpro manages and produces cost-of-service reserves for gas utility affiliate Questar Gas under the terms of the Wexpro agreements, comprehensive agreements with the states of Utah and Wyoming (see Note 10).
	Questar Gas is regulated by the PSCU and the PSCW. The Idaho Public Utilities Commission has contracted with the PSCU for rate oversight of Questar Gas operations in a small area of southeastern Idaho. Questar Pipeline is regulated by the FERC. These regulatory agencies establish rates for the sale, storage and transportation of natural gas. The regulatory agencies also regulate, among other things, the extension and enlargement or abandonment of jurisdictional natural gas facilities. Regulation is intended to permit the recovery, through rates, of the cost of service, including a return on investment.
Cash and Cash Equivalents	H. Cash and Cash Equivalents
	Cash equivalents consist principally of repurchase agreements with maturities of three months or less. In almost all cases, the repurchase agreements are highly liquid investments in overnight securities made through commercial bank accounts that result in available funds the next business day.
Notes Receivable and Notes Payable	I. Notes Payable to and Notes Receivable from Questar
	Notes payable to or receivable from Questar appearing in the financial statements and disclosures of Questar Gas and Questar Pipeline represent interest bearing demand notes for cash borrowed from Questar for use in operations or loaned to Questar until needed in operations. The funds are centrally managed by Questar. Amounts loaned to Questar earn an interest rate that is identical to the interest rate paid by the companies for borrowings from Questar.
Property, Plant and Equipment	J. Property, Plant and Equipment
	Property, plant and equipment balances are stated at historical cost. Maintenance and repair costs are expensed as incurred.
Cost-of-service gas and oil operations	Cost-of-service gas and oil operations
	The successful efforts method of accounting is used for cost-of-service reserves developed and produced by Wexpro for gas utility affiliate Questar Gas. Cost-of-service reserves are properties for which the operations and return on investment are subject to the Wexpro agreements (see Note 10). Under the successful efforts method, Wexpro capitalizes the costs of acquiring leaseholds, drilling development wells, drilling successful exploratory wells, and purchasing related support equipment and facilities. Geological and geophysical studies and other exploratory activities are expensed as incurred. Costs of production and general corporate activities are expensed in the period incurred. A gain or loss is generally recognized on assets as they are retired from service.
Contributions-in-aid-of construction	Contributions in aid of construction
	Customer contributions in aid of construction reduce plant unless the amounts are refundable to customers. Contributions for main-line extensions may be refundable to customers if additional customers connect to the main-line segment within five years. Refundable contributions are recorded as liabilities until refunded or the five-year period expires without additional customer connections. Amounts not refunded reduce plant. Capital expenditures in the Consolidated Statements of Cash Flows are reported net of non-refundable contributions. As a result of Questar Gas's recent Utah general rate case, effective March 1, 2014, the Company does not expect to record any new refundable customer contributions in aid of construction for Utah customers.
Depreciation, depletion and amortization	Depreciation, depletion and amortization
	Major categories of fixed assets in gas distribution, transportation and storage operations are grouped together and depreciated using a straight-line method. Gains and losses on asset disposals are recorded as adjustments in accumulated depreciation. The Company has not capitalized future abandonment costs on a majority of its long-lived gas distribution and transportation assets due to a lack of a legal obligation to restore the area surrounding abandoned assets. In these cases, the regulatory agencies have opted to leave retired facilities in the ground undisturbed rather than excavate and dispose of the assets. Depreciation rates for Questar Gas and Questar Pipeline are established through rate proceedings.
	Capitalized costs of development wells and leaseholds are amortized on a field-by-field basis using the unit-of-production method and the estimated proved developed or total proved gas and oil reserves. Oil and NGL volumes are converted to natural gas equivalents using the ratio of one barrel of crude oil, condensate or NGL to 6,000 cubic feet of natural gas. The Company capitalizes an estimate of the fair value of future abandonment costs associated with cost-of-service reserves and depreciates these costs using a unit-of-production method.
	The following represent average depreciation, depletion and amortization rates of the Company's capitalized costs:
		Year Ended December 31,
		2014			2013			2012
	Questar Gas distribution plant	2.7		%	2.7		%	2.8		%
	Cost-of-service gas and oil properties, per Mcfe	$	1.75		$	1.56		$	1.49
	Questar Pipeline transportation, storage and other energy services	3.2		%	3.4		%	3.4		%
	Questar Gas's depreciation rates include a component for the cost of plant removal. Accordingly, Questar Gas recognizes the cost of plant removal as depreciation expense. The related cost of removal accrual is reflected as a regulatory liability on the Questar Gas Balance Sheets (see Note 11). At the time property, plant and equipment is retired, removal expenses less salvage, are charged to the regulatory cost of plant removal liability.
Impairment of Long-Lived Assets	K. Impairment of Long-Lived Assets
	Proved gas and oil properties are evaluated on a field-by-field basis for potential impairment. Other properties are evaluated on a specific-asset basis or in groups of similar assets, as applicable. Impairment is indicated when a triggering event occurs and the sum of the estimated undiscounted future net cash flows of an evaluated asset is less than the asset's carrying value. Triggering events could include, but are not limited to, an impairment of gas and oil reserves caused by mechanical problems, faster-than-expected decline of reserves, lease-ownership issues, other-than-temporary decline in gas and oil prices, and changes in the utilization of pipeline assets. If impairment is indicated, fair value is estimated using a discounted cash flow approach that incorporates market interest rates or, if available, other market data. The amount of impairment loss recorded, if any, is the difference between the fair value of the asset and the current net book value. Cash flow estimates require forecasts and assumptions for many years into the future for a variety of factors, including commodity prices, commodity transportation rates and operating costs. Wexpro recorded a $2.0 million pre-tax abandonment and impairment charge for its share of the remaining investment in the Brady field in the second quarter of 2014. Questar Pipeline recorded an $80.6 million pre-tax impairment of the eastern segment of its Southern Trails Pipeline in the third quarter of 2013. See Note 16 for additional details. There were no impairments in 2012.
Goodwill and Other Intangible Assets	L. Goodwill
	Goodwill represents the excess of the amount paid over the fair value of net assets acquired in a business combination, and is not subject to amortization. Goodwill and indefinite-lived intangible assets are tested for impairment at least once a year or when a triggering event occurs. The Company evaluates whether it is more likely than not that the carrying value of a reporting unit is greater than its fair value using events and circumstances such as economic conditions, industry changes, financial performance, etc. Fair value is measured using actively traded market values of other comparable companies in the same businesses. If the fair value of the reporting unit exceeds its carrying value then goodwill is considered not to be impaired. If the carrying value of the business unit is greater than the fair value, an impairment of goodwill is recognized equal to the excess of the carrying amount of goodwill over its fair value.
Capitalized Interest and Allowance for Funds Used During Construction	M. Capitalized Interest and Allowance for Funds Used During Construction
	The Company capitalizes interest costs when applicable. The PSCU, PSCW and FERC require the capitalization of an allowance for funds used during construction (AFUDC) for rate-regulated plant and equipment. The Wexpro agreements require capitalization of AFUDC on cost-of-service gas and oil development projects.
Derivative Instruments and Hedging Activities	N. Derivative Instruments and Hedging Activities
	The Company may elect to designate a derivative instrument as a hedge of exposure to changes in fair value or cash flows. A derivative instrument qualifies as a hedge if all of the following tests are met:
	•	The item to be hedged exposes the Company to market risk.
	•	The derivative reduces the risk exposure and is designated as a hedge at the inception of the hedging relationship.
	•	At the inception of the hedge and throughout the hedge period, there is a high correlation between changes in the fair value of the derivative instrument and the fair value of the underlying hedged item.
	If the hedged exposure is a fair value exposure, the gain or loss on the derivative instrument is recognized in earnings in the period of the change together with the offsetting gain or loss from the change in fair value of the hedged item. If the hedged exposure is a cash flow exposure, the effective portion of the gain or loss on the derivative instrument is reported initially as a component of accumulated other comprehensive income (loss) (AOCI) and subsequently reclassified into earnings when the forecasted transaction affects earnings. Any amount excluded from the assessment of hedge effectiveness, as well as the ineffective portion of the gain or loss, is reported currently in earnings. When a derivative instrument is designated as a cash flow hedge of a forecasted transaction that becomes probable of not occurring, the gain or loss on the derivative is immediately reclassified into earnings from AOCI. See Note 6 for further discussion on derivatives and hedging.
Credit Risk	O. Credit Risk
	The Rocky Mountain region is the Company's primary market area. Exposure to credit risk may be affected by the concentration of customers in this region due to changes in economic or other conditions. Customers include individuals and numerous commercial and industrial enterprises that may react differently to changing conditions. Management believes that its credit-review procedures, loss reserves, customer deposits and collection procedures have adequately provided for usual and customary credit-related losses. Loss reserves are periodically reviewed for adequacy and may be established on a specific-case basis.
	Bad debt expense associated with accounts receivable amounted to $1.7 million in 2014, $0.2 million in 2013 and $1.2 million in 2012. The allowance for bad debts was $1.7 million at December 31, 2014 and 2013. Questar Gas's retail gas operations account for a majority of the bad debt expense. Questar Gas estimates bad debt expense as a percentage of general-service revenues with periodic adjustments. Uncollected accounts are generally written off six months after gas is delivered and interest is no longer accrued. Questar Gas recovers bad debt costs related to the gas-cost portion of rates in its Utah operations through a purchased-gas adjustment to rates.
Asset Retirement Obligations	P. Asset Retirement Obligations
	Questar records an asset retirement obligation (ARO) when there is a legal obligation associated with the retirement of a tangible long-lived asset. Questar's AROs apply primarily to abandonment costs associated with Wexpro gas and oil wells, production facilities and certain other properties. The Company has not capitalized future abandonment costs on a majority of its long-lived transportation and distribution assets because the Company does not have a legal obligation to restore the area surrounding abandoned assets. In these cases, the regulatory agencies have opted to leave retired facilities in the ground undisturbed rather than requiring the Company to excavate and dispose of the assets. The fair value of retirement costs is estimated by Company personnel based on abandonment costs of similar properties available to field operations and depreciated over the life of the related assets. Revisions to estimates result from material changes in the expected timing or amount of cash flows associated with AROs. Income or expense resulting from the settlement of ARO liabilities is included in net gain (loss) from asset sales on the Consolidated Statements of Income. The ARO liability is adjusted to present value each period through an accretion calculation using a credit-adjusted risk-free interest rate. See Note 4 for further discussion on AROs.
Income Taxes	Q. Income Taxes
	Questar and its subsidiaries file a consolidated federal income tax return. Questar Gas and Questar Pipeline account for income taxes on a separate return basis and record tax expenses and benefits as they are generated. Questar Gas and Questar Pipeline make payments to or receive payments from Questar for such tax expenses or benefits as they are generated on the consolidated income tax return. Deferred income taxes are recorded for the temporary differences arising between the book and tax carrying amounts of assets and liabilities. These differences create taxable or tax-deductible amounts for future periods. Questar Gas and Questar Pipeline use the deferral method to account for investment tax credits as required by regulatory commissions. The Company records interest earned on income tax refunds in interest and other income and records penalties and interest charged on tax deficiencies in interest expense.
	Accounting standards for income taxes specify the accounting for uncertainty in income taxes by prescribing a minimum recognition threshold for a tax position to be reflected in the financial statements. If recognized, the tax benefit is measured as the largest amount of tax benefit that is more-likely-than-not to be realized upon ultimate settlement. Management has considered the amounts and the probabilities of the outcomes that could be realized upon ultimate settlement and believes that it is more-likely-than-not that the Company's recorded income tax benefits will be fully realized. There were no unrecognized tax benefits at the beginning or end of the years ended December 31, 2014, 2013 or 2012. The 2014 federal income tax return has not been filed.
	In the 2012 tax year, Questar began participating in the IRS's Compliance Assurance Process (CAP). For the 2014 and 2015 tax years, Questar was accepted into the CAP Maintenance program. The CAP employs real-time resolution to improve federal tax compliance by resolving all or most tax positions prior to filing the related tax return. Successful conclusion of the CAP allows the IRS to achieve an acceptable level of assurance regarding the accuracy of the taxpayer's filed tax return and to eliminate or substantially reduce the need for a traditional examination. The CAP Maintenance program is administered by the IRS and indicates that the Company is a compliant taxpayer. The IRS has closed its review of all prior year tax returns.
	Federal tax statutes allowed taxpayers a deduction of bonus depreciation related to capital expenditures of 50% in 2012, 2013, and 2014. The effects of bonus depreciation and other significant book/tax timing differences resulted in a net operating loss (NOL) carryforward for federal income tax purposes of $26.9 million at December 31, 2012. Questar utilized the entire December 31, 2012 NOL on its 2013 federal income tax return. See Note 8 for further discussion on income taxes.
Earnings Per Share	R. Earnings Per Share
	Basic earnings per share (EPS) is computed by dividing net income by the weighted-average number of common shares outstanding during the reporting period, which includes vested undistributed restricted stock units (RSUs). Diluted EPS includes the potential increase in the number of outstanding shares that could result from the exercise of in-the-money stock options, the vesting of RSUs with forfeitable dividend equivalents and the distribution of performance shares that are part of the Company's Long-term Stock Incentive Plan (LTSIP), less shares repurchased under the treasury stock method. Restricted shares and RSUs with nonforfeitable dividends or dividend equivalents are participating securities for the computation of basic earnings per share under the two-class method. The application of the two-class method has an insignificant impact on the calculation of Questar's basic and diluted EPS. See Note 2 for further discussion on EPS.
Share-Based Compensation	S. Share-Based Compensation
	Questar may issue stock options, restricted shares, RSUs and performance shares to certain officers, employees and non-employee directors under the LTSIP. The Company uses the Black-Scholes-Merton mathematical model in estimating the fair value of stock options and the Monte Carlo simulation method in estimating the fair value of performance shares for accounting purposes. The granting of restricted shares and RSUs results in recognition of compensation cost measured at the grant-date market price. Questar uses an accelerated method in recognizing share-based compensation costs with graded vesting periods. See Note 12 for further discussion on share-based compensation.
Comprehensive Income	T. Comprehensive Income
	Beginning in 2013, the Company and its subsidiaries adopted accounting guidance issued in February 2013 that requires additional disclosures about reclassifications out of AOCI.
	Comprehensive income, as reported on Questar's Consolidated Statements of Comprehensive Income, is the sum of net income as reported on the Questar Consolidated Statements of Income and net other comprehensive income (loss) (OCI) as reported on the Questar Consolidated Statements of Common Shareholders' Equity. OCI includes recognition of the under-funded position of pension and other postretirement benefit plans, interest rate cash flow hedges, changes in the fair value of long-term investment, and the related income taxes. Income or loss is recognized when the pension and other postretirement benefit (OPB) costs are accrued, as the Company records interest expense for hedged interest payments and when the long-term investment is sold or otherwise realized.
	Comprehensive income, as reported on Questar Pipeline's Consolidated Statements of Comprehensive Income, is the sum of net income as reported on the Questar Pipeline Consolidated Statements of Income and net OCI as reported on the Questar Pipeline Consolidated Statements of Common Shareholder's Equity. OCI includes interest rate cash flow hedges and the related income taxes. Income or loss is recognized as the company records interest expense for hedged interest payments. See Note 3 for additional information related to OCI and AOCI.
Business Segments	U. Business Segments
	Line of business information is presented according to senior management's basis for evaluating performance considering differences in the nature of products, services and regulation, among other factors. Certain intersegment sales include intercompany profit
Recent Accounting Developments	V. Recent Accounting Developments
	In May 2014, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606) (ASU 2014-09), to supersede nearly all existing revenue recognition guidance under U.S. GAAP. The core principle of ASU 2014-09 is to recognize revenues when promised goods or services are transferred to customers in an amount that reflects the consideration that is expected to be received for those goods or services. ASU 2014-09 is effective for the Company beginning on January 1, 2017 using either of two methods: (i) retrospective application to each prior reporting period presented with the option to elect certain practical expedients as defined in the ASU; or (ii) retrospective application with the cumulative effect of initially applying the guidance recognized at the date of initial application and providing certain additional disclosures as defined in the ASU. Early adoption is not permitted. The Company is currently evaluating the ASU's impact on its financial position, results of operations and cash flows, as well as which transition approach it will take.
Reclassifications	W. Reclassifications
	Certain reclassifications were made to prior year financial statements and notes to conform to the 2014 presentation

Summary_of_Significant_Account2

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Line Items]
Cost of sales detail	The details of Questar's consolidated cost of sales are as follows:
		Year Ended December 31,
		2014			2013			2012
		(in millions)
	Questar Gas
	Gas purchases	$	136.5		$	186.6		$	104.2
	Operator service fee	349.7			294.6			274
	Transportation and storage	79.6			80.1			79.6
	Gathering	21			18.8			20.5
	Royalties	60.1			44.3			32
	Storage (injection) withdrawal, net	(1.1		)	(0.8		)	1.9
	Purchased-gas account adjustment	(45.8		)	22			16.1
	Other	4.8			5			5
	Total Questar Gas cost of natural gas sold	604.8			650.6			533.3
	Elimination of Questar Gas cost of natural gas sold - affiliated companies	(423.4		)	(370.9		)	(347.7		)
	Total Questar Gas cost of natural gas sold - unaffiliated parties	181.4			279.7			185.6
	Questar Pipeline
	Total Questar Pipeline cost of sales	4			6.1			6.7
	Other cost of sales	0.9			0.1			—
	Total cost of sales	$	186.3		$	285.9		$	192.3
Average depreciation, depletion and amortization rates of the Company's capitalized costs	The following represent average depreciation, depletion and amortization rates of the Company's capitalized costs:
		Year Ended December 31,
		2014			2013			2012
	Questar Gas distribution plant	2.7		%	2.7		%	2.8		%
	Cost-of-service gas and oil properties, per Mcfe	$	1.75		$	1.56		$	1.49
	Questar Pipeline transportation, storage and other energy services	3.2		%	3.4		%	3.4		%
Amounts recorded in the income statement for the capitalization of funds used during construction and interest costs	Amounts recorded in the Consolidated Statements of Income for the capitalization of AFUDC and interest costs are disclosed in the table below:
		Year Ended December 31,
		2014			2013			2012
		(in millions)
	AFUDC (recorded as an increase in interest and other income)
	Questar Gas	$	0.9		$	—		$	—
	Wexpro	0.8			4.6			2.1
	Questar Pipeline	1.1			1.7			0.2
	Total AFUDC	$	2.8		$	6.3		$	2.3
	Capitalized interest costs (recorded as a reduction of interest expense)
	Questar Gas	$	0.5		$	0.2		$	0.1
	Questar Pipeline	0.5			0.6			0.1
	Total capitalized interest costs	$	1		$	0.8		$	0.2

Earnings_Per_Share_Tables

Earnings Per Share (Tables)	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Schedule Of Weighted Average Number Of Shares Basic And Diluted	A reconciliation of the components of basic and diluted shares used in the EPS calculation follows:
		Year Ended December 31,
		2014	2013	2012
		(in millions)
	Weighted-average basic common shares outstanding	175.8	175.4	176.5
	Potential number of shares issuable under the Company's LTSIP	0.3	0.6	1
	Weighted-average diluted common shares outstanding	176.1	176	177.5

Accumulated_Other_Comprehensiv1

Accumulated Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2014
Other Comprehensive Income [Abstract]
Schedule of Accumulated Other Comprehensive Income (Loss)	Details of the changes in the components of AOCI, net of income taxes, as reported in Questar's Consolidated Statements of Common Shareholders' Equity, are shown in the tables below. The tables also disclose details of income taxes related to each component of OCI:
		Pension									OPB			Interest rate cash flow hedges			Long-term invest.			Total
		Actuarial loss			Prior service cost			Total			Actuarial loss
		(in millions)
	Balances at December 31, 2012	$	(202.4	)	$	(1.1	)	$	(203.5	)	$	(19.7	)	$	(23.2	)	$	0.1		$	(246.3	)
	OCI before reclassifications	123.4			—			123.4			12.4			—			(0.1		)	135.7
	Reclassified from AOCI(1)	28.9			1.1			30			3			0.5			—			33.5
	Income taxes
	OCI before reclassifications	(47.2		)	—			(47.2		)	(4.7		)	—			0.1			(51.8		)
	Reclassified from AOCI(2)	(11.1		)	(0.4		)	(11.5		)	(1.2		)	(0.2		)	—			(12.9		)
	Total income taxes	(58.3		)	(0.4		)	(58.7		)	(5.9		)	(0.2		)	0.1			(64.7		)
	Net other comprehensive income	94			0.7			94.7			9.5			0.3			—			104.5
	Balances at December 31, 2013	(108.4		)	(0.4		)	(108.8		)	(10.2		)	(22.9		)	0.1			(141.8		)
	OCI before reclassifications	(111.5		)	—			(111.5		)	(2.1		)	—			—			(113.6		)
	Reclassified from AOCI(1)	15.4			0.6			16			0.7			0.5			—			17.2
	Income taxes
	OCI before reclassifications	42.6			—			42.6			0.8			—			—			43.4
	Reclassified from AOCI(2)	(5.9		)	(0.2		)	(6.1		)	(0.3		)	(0.1		)	—			(6.5		)
	Total income taxes	36.7			(0.2		)	36.5			0.5			(0.1		)	—			36.9
	Net other comprehensive income (loss)	(59.4		)	0.4			(59.0		)	(0.9		)	0.4			—			(59.5		)
	Balances at December 31, 2014	$	(167.8	)	$	—		$	(167.8	)	$	(11.1	)	$	(22.5	)	$	0.1		$	(201.3	)
	(1) Interest rate cash flow hedge amounts are included in their entirety as charges to interest expense on the Consolidated Statements of Income.
	(2) Income tax reclassifications related to interest rate cash flow hedge amounts are included in their entirety as credits to income taxes on the Consolidated Statements of Income.
Income taxes allocated to each component of consolidated OCI	Income taxes allocated to each component of Questar's consolidated OCI for the year ended December 31, 2012 are shown below. Expenses are in parentheses.
		Year Ended December 31,
		2012
		(in millions)
	Change in unrecognized actuarial loss	$	13.6
	Change in unrecognized prior service cost	(1.0		)
	Interest rate cash flow hedges	(0.2		)
	Change in fair value of long-term investment	(0.1		)
	Income taxes	$	12.3

Asset_Retirement_Obligations_T

Asset Retirement Obligations (Tables)	12 Months Ended
	Dec. 31, 2014
Asset Retirement Obligation [Abstract]
Changes in AROs	Changes in Questar's AROs from the Consolidated Balance Sheets were as follows:
		Year Ended December 31,
		2014			2013
		(in millions)
	AROs at beginning of year	$	67.7		$	67.2
	Accretion	3.4			3.3
	Liabilities incurred	3.3			6
	Revisions in estimated cash flows	(2.7		)	(6.9		)
	Liabilities settled	(2.4		)	(1.9		)
	AROs at end of year	$	69.3		$	67.7
Asset Retirement Obligations by line of business	Questar's consolidated AROs by line of business are summarized in the table below:
		December 31,
		2014			2013
		(in millions)
	Questar Gas	$	0.6		$	0.6
	Wexpro	66.3			64.9
	Questar Pipeline	2.3			2.2
	Other	0.1			—
	Total	$	69.3		$	67.7

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value, by Balance Sheet Grouping	The following table discloses the carrying amount, estimated fair value and level within the fair value hierarchy of certain financial instruments not disclosed in other notes to Questar's financial statements in this Annual Report:
		Hierarchy Level of Fair Value Estimates	Carrying Amount		Estimated Fair Value		Carrying Amount		Estimated Fair Value
			December 31, 2014				December 31, 2013
			(in millions)
	Financial assets
	Cash and cash equivalents	1	$	32	$	32	$	16	$	16
	Long-term investment	1	15.7		15.7		17.3		17.3
	Financial liabilities
	Short-term debt	1	347		347		276		276
	Long-term debt, including current portion	2	1,245.20		1,356.10		1,247.10		1,310.80
Questar Gas [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value, by Balance Sheet Grouping	The following table discloses the carrying amount, estimated fair value and level within the fair value hierarchy of certain financial instruments not disclosed in other notes to Questar Gas's financial statements in this Annual Report:
		Hierarchy Level of Fair Value Estimates	Carrying Amount		Estimated Fair Value		Carrying Amount		Estimated Fair Value
			December 31, 2014				December 31, 2013
			(in millions)
	Financial assets
	Cash and cash equivalents	1	$	19.8	$	19.8	$	8.8	$	8.8
	Financial liabilities
	Notes payable to Questar	1	119.3		119.3		17.7		17.7
	Long-term debt	2	534.5		607.2		534.5		568
Questar Pipeline [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value, by Balance Sheet Grouping	The following table discloses the carrying amount, estimated fair value and level within the fair value hierarchy of certain financial instruments not disclosed in other notes to Questar Pipeline's financial statements in this Annual Report:
		Hierarchy Level of Fair Value Estimates	Carrying Amount		Estimated Fair Value		Carrying Amount		Estimated Fair Value
			December 31, 2014				December 31, 2013
			(in millions)
	Financial assets
	Cash and cash equivalents	1	$	7.4	$	7.4	$	2.7	$	2.7
	Notes receivable from Questar	1	40.1		40.1		29.4		29.4
	Financial liabilities
	Long-term debt, including current portion	2	458.8		495.5		458.9		486

Debt_Tables

Debt (Tables)	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Schedule of Short-term Debt	The details of short-term debt are as follows:
		December 31,
		2014			2013
		(in millions)
	Commercial paper with various interest rates	$	347		$	276
	Weighted-average interest rate at end of year	0.23		%	0.27		%
Schedule of Notes Payable	The following table details the notes payable to Questar from Questar Gas and the associated interest rates. There were no notes payable to Questar from Questar Pipeline at December 31, 2014 or 2013.
		December 31,
		2014			2013
		(in millions)
	Questar Gas
	Notes payable to Questar	$	119.3		$	17.7
	Interest rate at end of year	0.25		%	0.3		%
Schedule of Debt	The details of long-term debt are as follows:
		December 31,
		2014			2013
		(in millions)
	Questar Corporation
	2.75% Notes due 2016	$	250		$	250
	Questar Gas
	5.31% and 6.85% Medium-term Notes due 2017 and 2018	84.5			84.5
	6.30% Notes due 2018	50			50
	2.98% Notes due 2024	40			40
	3.28% Notes due 2027	110			110
	7.20% Notes due 2038	100			100
	4.78% Notes due 2043	90			90
	4.83% Notes due 2048	60			60
	Total Questar Gas long-term debt	534.5			534.5
	Questar Pipeline
	6.45% and 6.48% Medium-term Notes due 2015 and 2018	30.1			30.1
	5.83% Notes due 2018	250			250
	4.875% Notes due 2041	180			180
	Total Questar Pipeline long-term debt	460.1			460.1
	Total long-term debt outstanding	1,244.60			1,244.60
	Less current portion	(25.1		)	—
	Less unamortized debt discount	(1.9		)	(2.1		)
	Plus unamortized debt premium	0.5			0.7
	Plus fair value hedge adjustment	2			3.9
	Total long-term debt, less current portion	$	1,220.10		$	1,247.10
Schedule of Debt Maturities	The aggregate maturities of Questar Corporation, Questar Gas and Questar Pipeline long-term debt for the next five years are as follows:
		Questar Corporation			Questar			Questar Pipeline		Total
					Gas
		Years Ending December 31,
		(in millions)
	2015	$	—		$	—		$	25.1	$	25.1
	2016	250			—			—		250
	2017	—			14.5			—		14.5
	2018	—			120			255		375
	2019	—			—			—		—

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2014
Components of income tax expense	The components of income tax expense were as follows:
		Year Ended December 31,
		2014				2013				2012
		(in millions)
	Federal
	Current	$	69.6			$	66.5			$	1.3
	Deferred	47.5				18.4				112
	State
	Current	6.9				10.7				—
	Deferred	2.1				5.9				3.6
	Deferred investment tax credits recognized	(0.2		)		(0.2		)		(0.4		)
	Total income tax expense	$	125.9			$	101.3			$	116.5
Difference between statutory federal income tax rate and the Company's effective income tax rate	The difference between the statutory federal income tax rate and the Company's effective income tax rate is explained as follows:
		Year Ended December 31,
		2014			2013			2012
	Federal income taxes statutory rate	35	%		35	%		35	%
	Increase (decrease) in rate as a result of:
	State income taxes, net of federal income tax benefit	1.6			4			1
	Tax benefits from dividends paid to employee stock plan	(1.0	)		(1.0	)		(0.5	)
	Other	0.1			0.6			—
	Effective income tax rate	35.7	%		38.6	%		35.5	%
Significant components of deferred income taxes	Significant components of Questar's deferred income taxes were as follows:
		December 31,
		2014				2013
		(in millions)
	Deferred income taxes - noncurrent
	Deferred tax liabilities
	Property, plant and equipment	$	827			$	797.5
	Employee benefits	52				30.2
	Other	0.5				4.9
	Deferred tax liabilities - noncurrent	879.5				832.6
	Deferred tax assets
	Asset retirement obligations	22.5				22.7
	Pension and other postretirement benefits	110.8				73.7
	Deferred compensation	15.7				14.5
	Hedging activities	13.2				13.4
	State tax credits	3.3				3.4
	Valuation allowance	(1.6		)		(2.3		)
	Deferred tax assets, net of allowance - noncurrent	163.9				125.4
	Net deferred income tax liability - noncurrent	$	715.6			$	707.2
	Deferred income taxes - current
	Deferred tax assets - current	$	15.7			$	10.1
	Deferred tax liabilities - current	9.9				0.4
	Net deferred income tax asset - current	$	5.8			$	9.7
Questar Gas [Member]
Components of income tax expense	The components of income tax expense were as follows:
		Year Ended December 31,
		2014				2013				2012
		(in millions)
	Federal
	Current	$	(11.9	)		$	6			$	(18.7	)
	Deferred	42.4				23.5				45.1
	State
	Current	(1.9		)		0.6				—
	Deferred	3.6				2				1.2
	Deferred investment tax credit recognized	(0.2		)		(0.2		)		(0.4		)
	Total income tax expense	$	32			$	31.9			$	27.2
Difference between statutory federal income tax rate and the Company's effective income tax rate	The difference between the statutory federal income tax rate and Questar Gas's effective income tax rate is explained as follows:
		Year Ended December 31,
		2014			2013			2012
	Federal income taxes statutory rate	35	%		35	%		35	%
	Increase (decrease) in rate as a result of:
	State income taxes, net of federal income tax benefit	1.3			2.1			1
	Amortization of investment tax credits related to rate-regulated assets	(0.2	)		(0.3	)		(0.4	)
	Other	0.6			0.9			1
	Effective income tax rate	36.7	%		37.7	%		36.6	%
Significant components of deferred income taxes	Significant components of Questar Gas's deferred income taxes were as follows:
		December 31,
		2014				2013
		(in millions)
	Deferred income taxes - noncurrent
	Deferred tax liabilities
	Property, plant and equipment	$	354.4			$	322.6
	Employee benefits	23.5				14
	Other	0.5				4.9
	Deferred tax liabilities - noncurrent	378.4				341.5
	Deferred tax assets
	Deferred compensation	0.9				0.8
	Deferred tax assets - noncurrent	0.9				0.8
	Net deferred income tax liability - noncurrent	$	377.5			$	340.7
	Deferred income taxes - current
	Deferred tax assets - current	$	3.6			$	3.3
	Deferred tax liabilities - current	9.9				0.4
	Net deferred income tax asset (liability) - current	$	(6.3	)		$	2.9
Questar Pipeline [Member]
Components of income tax expense	The components of income tax expense were as follows:
		Year Ended December 31,
		2014				2013				2012
		(in millions)
	Federal
	Current	$	21.9			$	20.2			$	(5.4	)
	Deferred	11.6				(15.2		)		40.4
	State
	Current	1				1.3				1.6
	Deferred	1.1				0.7				0.7
	Total income tax expense	$	35.6			$	7			$	37.3
Difference between statutory federal income tax rate and the Company's effective income tax rate	The difference between the statutory federal income tax rate and Questar Pipeline's effective income tax rate is explained as follows:
		Year Ended December 31,
		2014			2013			2012
	Federal income taxes statutory rate	35	%		35	%		35	%
	Increase in rate as a result of:
	State income taxes, net of federal income tax benefit	1.5			8.5			1.6
	Other	0.5			2.6			—
	Effective income tax rate	37	%		46.1	%		36.6	%
Significant components of deferred income taxes	Significant components of Questar Pipeline's deferred income taxes were as follows:
		December 31,
		2014				2013
		(in millions)
	Deferred income taxes - noncurrent
	Deferred tax liabilities
	Property, plant and equipment	$	246.1			$	237.1
	Employee benefits	10.8				7.4
	Deferred tax liabilities - noncurrent	256.9				244.5
	Deferred tax assets
	Deferred compensation	2				1.8
	Hedging activities	13.2				13.4
	State tax credits	0.7				0.7
	Valuation allowance	(0.4		)		—
	Deferred tax assets, net of allowance - noncurrent	15.5				15.9
	Net deferred income tax liability - noncurrent	$	241.4			$	228.6
	Net deferred income tax asset - current	$	1.9			$	1.8

Contingencies_Commitments_and_1

Contingencies, Commitments and Leases (Tables)	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Schedule of Future Minimum Lease Payments for Capital Lease	Future minimum lease payments for the five years following 2014 and the years thereafter are shown in the table below. Also shown is the present value of minimum lease payments at December 31, 2014, which is reflected on the Consolidated Balance Sheets as current and noncurrent capital lease obligations of $1.0 million and $37.4 million, respectively.
		Years Ending December 31,
		(in millions)
	2015	$	3.1
	2016	3.5
	2017	3.6
	2018	3.7
	2019	3.8
	After 2019	41
	Total minimum lease payments	58.7
	Less: amount representing interest	(20.3		)
	Present value of minimum lease payments at December 31, 2014	$	38.4

Wexpro_and_Wexpro_II_Agreement1

Wexpro and Wexpro II Agreements (Tables)	12 Months Ended
	Dec. 31, 2014
Wexpro Agreement [Abstract]
Net investment base, yearly average rate of return and oil and NGL income sharing	Wexpro's net investment base, yearly average rate of return, and oil and NGL income shared with Questar Gas are shown in the table below:
		Year Ended December 31,
		2014			2013			2012
	Wexpro net investment base at December 31, (in millions)	$	649		$	589.7		$	531.1
	Average annual rate of return (after tax)	17.9		%	19.7		%	19.9		%
	Oil and NGL income sharing (in millions)	$	—		$	0.6		$	2.5

Rate_Regulation_Tables

Rate Regulation (Tables)	12 Months Ended
	Dec. 31, 2014
Regulated Operations [Abstract]
Details of regulatory assets and liabilities	The following table details regulatory assets and liabilities:
		December 31, 2014				December 31, 2013
		Current		Noncurrent		Current		Noncurrent
		(in millions)
	Regulatory assets:
	Questar Gas
	Purchased-gas adjustment	$	39.2	$	—	$	—	$	—
	Demand-side management	—		—		11.2		—
	Contract withholding	13.6		—		8.1		—
	Deferred cost-of-service gas charges	25.5		9.3		10.9		2
	Cost of reacquired debt	—		4.3		—		4.8
	Pipeline integrity costs	—		7.7		—		9.3
	Total Questar Gas regulatory assets	78.3		21.3		30.2		16.1
	Questar Pipeline
	Gas imbalance	1.2		—		5.3		—
	Revenue sharing	0.1		—		0.3		—
	Cost of reacquired debt	—		2.2		—		2.6
	Income taxes recoverable from customers	—		0.3		—		0.6
	Other	—		1.2		—		1.2
	Total Questar Pipeline regulatory assets	1.3		3.7		5.6		4.4
	Total regulatory assets	$	79.6	$	25	$	35.8	$	20.5
	Regulatory liabilities:
	Questar Gas
	Purchased-gas adjustment	$	—	$	—	$	7.4	$	—
	Demand-side management	0.3		—		—		—
	CET	12.1		—		3.3		—
	Cost of plant removal	—		60.7		—		52.7
	Income taxes refundable to customers	—		0.2		—		0.3
	Other	0.1		—		0.4		—
	Total Questar Gas regulatory liabilities	12.5		60.9		11.1		53
	Questar Pipeline
	Gas imbalance	0.8		—		3		—
	Revenue sharing	0.1		—		—		—
	Postretirement medical	—		9		—		8.2
	Total Questar Pipeline regulatory liabilities	0.9		9		3		8.2
	Total regulatory liabilities	$	13.4	$	69.9	$	14.1	$	61.2

ShareBased_Compensation_Tables

Share-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Transactions involving stock options under the terms of the LTSIP	Stock option transactions and balances under the terms of the LTSIP are summarized below:
		Options Outstanding			Exercise Price Range								Weighted-Average Exercise Price
	Balance at December 31, 2013	717,840			$	7.84		-	$	17.35			$	11.84
	Exercised	(282,333	)		7.84			-	13.24				10.88
	Balance at December 31, 2014	435,507			$	11.4		-	$	17.35			$	12.46
Options held under the LTSIP, by exercise price range
	Options Outstanding															Options Exercisable
	Range of exercise prices								Number outstanding at Dec. 31, 2014			Weighted-average remaining term in years		Weighted-average exercise price		Number exercisable at Dec. 31, 2014	Weighted-average exercise price
	$	11.4			$	11.4			239,174			1.2		$	11.4	239,174	$	11.4
	$	13.1		-	17.35				196,333			2		13.75		196,333	13.75
									435,507			1.5		$	12.46	435,507	$	12.46
Transactions involving restricted shares and RSUs under the terms of the LTSIP	Transactions involving RSUs under the terms of the LTSIP for the year ended December 31, 2014, are summarized below:
		RSUs Outstanding			Price Range								Weighted-Average Price
	Balance at December 31, 2013	316,268			$	22.17		-	$	25.44			$	23.62
	Granted	351,066			22.14			-	24.7				23.59
	Vested	(131,007	)		22.17			-	25.44				23.64
	Forfeited	(10,225	)		23.6			-	23.62				23.6
	Balance at December 31, 2014	526,102			$	22.14		-	$	24.7			$	23.6
	Transactions involving restricted shares under the terms of the LTSIP for the year ended December 31, 2014, are summarized below:
		Restricted Shares Outstanding			Price Range								Weighted- Average Price
	Balance at December 31, 2013	392,327			$	15.22		-	$	21.53			$	18.28
	Vested	(289,312	)		15.22			-	21.53				17.88
	Forfeited	(661	)		19.39			-	20.9				20
	Balance at December 31, 2014	102,354			$	17.97		-	$	21.53			$	19.41
Calculated fair value of performance shares granted and major assumptions used in the Monte Carlo simulation	The estimated fair value of performance shares granted and major assumptions used in the simulation at the date of grant are listed below:
		2014				2013				2012
		Performance Share Input Variables
	Fair value of performance shares at grant date	$	31.07			$	39.62			$	25.42
	Risk-free interest rate	0.67		%		0.4		%		0.35		%
	Expected price volatility	18.6		%		19		%		22		%
	Expected dividend yield	3.05		%		2.88		%		3.35		%
	Expected life in years	2.9				2.9				2.9
Transactions involving performance shares under the terms of the LTSIP	Transactions involving performance shares under the terms of the LTSIP for the year ended December 31, 2014, are summarized below:
		Target Number of Performance Shares Outstanding			Grant-Date Fair Value Range								Weighted-Average Grant-Date Fair Value
	Balance at December 31, 2013	387,879			$	18.23		-	$	39.62			$	27.61
	Granted	139,611			31.07				31.07				31.07
	Payout adjustment(1)	(76,657	)		18.23				18.23				18.23
	Distributed(1)	(51,104	)		18.23				18.23				18.23
	Forfeited	(8,480	)		25.42			-	39.62				33.33
	Balance at December 31, 2014(2),(3)	391,249			$	25.42		-	$	39.62			$	31.78
	(1) Actual shares and cash distributed were determined by multiplying the target shares by 0.40 to reflect Questar's final total shareholder return ranking relative to the specified peer companies for the performance period from January 1, 2011 through December 31, 2013. The total amount paid for the cash half of the distribution was $0.6 million.
	(2) The total undistributed balance consists of 262,979 of share-settled target shares and 128,270 of cash-settled target shares.
	(3) The total undistributed balance includes 134,778 vested target shares that are expected to be distributed in the first quarter of 2015 using a payout adjustment multiplier of 1.00. The multiplier reflects Questar's final total shareholder return ranking relative to the specified peer companies for the performance period from January 1, 2012 through December 31, 2014. The distribution will be half in shares, half in cash. These target shares are included in the amounts described in note (2), immediately above.
Questar Pipeline and Questar Gas [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options held under the LTSIP, by exercise price range	The following table summarizes the stock options held under the LTSIP by Questar Pipeline officers and employees at December 31, 2014. There were no stock options held under the LTSIP by Questar Gas officers or employees at December 31, 2014.
	Options Outstanding															Options Exercisable
	Range of exercise prices								Number outstanding at Dec. 31, 2014			Weighted-average remaining term in years		Weighted-average exercise price		Number exercisable at Dec. 31, 2014	Weighted-average exercise price
	Questar Pipeline
	$	11.4		-	$	13.1			58,933			1.5		$	11.95	58,933	$	11.95
Transactions involving restricted shares and RSUs under the terms of the LTSIP	The following table summarizes the restricted shares held under the LTSIP by Questar Gas and Questar Pipeline officers and employees at December 31, 2014. The weighted-average remaining vesting periods of unvested restricted shares at December 31, 2014, for Questar Gas and Questar Pipeline were 2 months and 3 months, respectively.
		Restricted			Price Range								Weighted-Average Price
		Shares Outstanding
	Questar Gas	12,635			$	19.39			$	19.39			$	19.39
	Questar Pipeline	25,269			$	19.39		-	$	19.56			$	19.42
	The following table summarizes the RSUs held under the LTSIP by Questar Gas and Questar Pipeline officers and employees at December 31, 2014. The weighted-average remaining vesting period of unvested RSUs at December 31, 2014, for Questar Gas and Questar Pipeline was 12 months.
		RSUs Outstanding			Price Range								Weighted-Average Price
	Questar Gas	82,351			$	23.09		-	$	23.62			$	23.61
	Questar Pipeline	95,841			$	23.09		-	$	23.62			$	23.59
Transactions involving performance shares under the terms of the LTSIP	The following table summarizes the target number of performance shares held under the LTSIP by Questar Gas and Questar Pipeline officers at December 31, 2014. The weighted-average remaining vesting period of unvested performance shares at December 31, 2014, for Questar Gas and Questar Pipeline was 18 months.
		Target Number of Performance Shares Outstanding			Grant-Date Fair Value Range								Weighted-Average Grant-Date Fair Value
	Questar Gas	33,826			$	25.42		-	$	39.62			$	31.73
	Questar Pipeline	59,956			$	25.42		-	$	39.62			$	31.59

Employee_Benefits_Tables

Employee Benefits (Tables)	12 Months Ended
	Dec. 31, 2014
Compensation and Retirement Disclosure [Abstract]
Schedule of pension and other postretirement benefit plan asset allocation	Qualified pension and other postretirement benefit plan assets were invested as follows:
		Actual Allocation						Policy Range
		December 31,						December 31,
		2014			2013			2014					2013
	Total domestic equity securities	38	%		38	%		35	%	-	45	%	35	%	-	45	%
	Foreign equity securities
	Developed market foreign equity securities	18	%		20	%
	Emerging market foreign equity securities	4	%		5	%
	Total foreign securities	22	%		25	%		25	%	-	35	%	25	%	-	35	%
	Debt securities
	Investment-grade intermediate-term debt	7	%		7	%
	Investment-grade long-term debt	14	%		11	%
	Below-investment-grade debt	9	%		9	%
	Total debt securities	30	%		27	%		25	%	-	35	%	25	%	-	35	%
	Inflation protection securities	9	%		9	%		—	%	-	10	%	—	%	-	10	%
	Cash and short-term investments	1	%		1	%		—	%	-	3	%	—	%	-	3	%
Pension and postretirement benefit assets fair value by level	The following tables set forth, by level within the fair value hierarchy, qualified pension and other postretirement benefit plan assets at fair value as of December 31, 2014 and 2013:
		December 31, 2014
		Level 1				Level 2				Total
		(in millions)
	Corporate bonds	$	—			$	1.5			$	1.5
	U.S. government securities	—				12.9				12.9
	Registered investment companies:
	Fixed income funds	114.9				—				114.9
	Inflation protection fund	34.5				—				34.5
	Domestic equity fund	1.6				—				1.6
	Commingled funds:
	Cash equivalent funds	—				6.7				6.7
	Domestic equity index fund	—				275.8				275.8
	Foreign equity growth fund	—				41.2				41.2
	Foreign equity index funds	—				77.8				77.8
	Corporate debt funds	—				91				91
	Inflation protection fund	—				33.1				33.1
	Foreign equity growth 103-12 investment entity	—				46.2				46.2
	Total	$	151			$	586.2			$	737.2
		December 31, 2013
		Level 1				Level 2				Total
		(in millions)
	Corporate bonds	$	—			$	1.7			$	1.7
	U.S. government securities	—				1.6				1.6
	Registered investment companies:
	Fixed income funds	110.7				—				110.7
	Inflation protection fund	29.4				—				29.4
	Domestic equity fund	2				—				2
	Commingled funds:
	Cash equivalent funds	—				3.5				3.5
	Domestic equity index fund	—				256.2				256.2
	Foreign equity growth fund	—				42.9				42.9
	Foreign equity index funds	—				76				76
	Corporate debt funds	—				72.1				72.1
	Inflation protection fund	—				28.8				28.8
	Foreign equity growth 103-12 investment entity	—				47.1				47.1
	Total	$	142.1			$	529.9			$	672
Assumptions used to calculate benefit obligations	The qualified pension projected benefit obligation was measured using the following assumptions at December 31:
		2014			2013
	Discount rate	4.1	%		4.9	%
	Rate of increase in compensation	5.5			5.5
	Long-term return on assets	7			7.25
	The nonqualified pension projected benefit obligation was measured using the following assumptions at December 31:
		2014			2013
	Discount rate	2.9	%		3.3	%
	Rate of increase in compensation	5.5			5.5
	The postretirement accumulated benefit obligation was measured using the following assumptions at December 31:
		2014						2013
	Discount rate	4				%		4.7				%
	Long-term return on assets	7						7.25
	Health-care inflation rate	7.5						8
		decreasing to						decreasing to
		4.5	%	by	2021			4.5	%	by	2021
Changes in benefit obligation, changes in fair value of plan assets, and underfunded status	Plan obligations and fair value of all plan assets are shown in the following table:
		Pension								Other Postretirement Benefits
		Year Ended December 31,								Year Ended December 31,
		2014				2013				2014			2013
		(in millions)
	Change in benefit obligation
	Benefit obligation at beginning of year	$	702.7			$	760.3			$	86.3		$	94.4
	Service cost	11.8				13.9				0.6			0.7
	Interest cost	33.4				30.6				3.8			3.7
	Change in plan assumptions	109.1				(73.6		)		4.9			(8.6		)
	Actuarial (gain)	(0.7		)		(7.7		)		(3.0		)	(0.5		)
	Benefits paid	(22.3		)		(20.8		)		(1.8		)	(3.4		)
	Benefit obligation at end of year	834				702.7				90.8			86.3
	Change in plan assets
	Fair value of plan assets at beginning of year	628.2				510.8				43.8			37.6
	Actual gain on plan assets	40.4				80.2				2.9			5.8
	Company contributions to the plan	43.9				58				2.1			3.8
	Benefits paid	(22.3		)		(20.8		)		(1.8		)	(3.4		)
	Fair value of plan assets at end of year	690.2				628.2				47			43.8
	Underfunded status (current and long-term)	$	(143.8	)		$	(74.5	)		$	(43.8	)	$	(42.5	)
Estimated benefit plan payments for the next five years and the subsequent five years aggregated	Estimated benefit plan payments for the five years following 2014 and the subsequent five years aggregated are as follows:
		Pension				Other Postretirement Benefits
		Years Ending December 31,
		(in millions)
	2015	$	67.3			$	4.3
	2016	52.6				4.5
	2017	47.2				4.6
	2018	63				4.8
	2019	53.6				5
	2020 through 2024	265				26.3
Net pension and other postretirement benefit cost components	The components of the net pension and other postretirement benefit costs are as follows. The net pension cost includes both the qualified and nonqualified pension plans.
		Pension											Other Postretirement Benefits
		Year Ended December 31,											Year Ended December 31,
		2014				2013				2012			2014				2013			2012
		(in millions)
	Service cost	$	11.8			$	13.9			$	13.2		$	0.6			$	0.7		$	0.7
	Interest cost	33.4				30.6				31.1			3.8				3.7			3.8
	Expected return on plan assets	(43.6		)		(38.1		)		(31.8		)	(3.0		)		(2.6		)	(2.3		)
	Prior service and other costs	0.6				1.1				1.1			—				—			1.6
	Recognized net actuarial loss	15.4				28.9				24.9			0.7				3			2.5
	Accretion of regulatory liability	—				—				—			0.8				0.5			0.3
	Net periodic cost	$	17.6			$	36.4			$	38.5		$	2.9			$	5.3		$	6.6
Assumptions used to calculate net benefit costs	Assumptions at January 1, used to calculate the qualified net pension cost for the years, were as follows:
		2014			2013			2012
	Discount rate	4.9	%		4.2	%		4.8	%
	Rate of increase in compensation	5.5			5.5			5.5
	Long-term return on assets	7.25			7.25			7.25
	Assumptions at January 1, used to calculate the nonqualified net pension cost for the years, were as follows:
		2014			2013			2012
	Discount rate	3.3	%		2.4	%		3.3	%
	Rate of increase in compensation	5.5			5.5			5.5
	Assumptions at January 1, used to calculate the net postretirement benefit cost for the years, were as follows:
		2014						2013					2012
	Discount rate	4.7				%		4				%	4.4				%
	Long-term return on assets	7.25						7.25					7.25
	Health-care inflation rate	8						8.5					7
		decreasing to						decreasing to					decreasing to
		4.5	%	by	2021			4.5	%	by	2021		5	%	by	2014
Subsidiary net cost and contribution information	Pension and other postretirement benefit net cost and plan contribution information for Questar Gas and Questar Pipeline are shown below:
		Pension											Other Postretirement Benefits
		Year Ended December 31,											Year Ended December 31,
		2014				2013				2012			2014				2013			2012
		(in millions)
	Questar Gas
	Net periodic cost	$	8.5			$	18.1			$	19.7		$	0.8			$	2.4		$	3.3
	Share of total plan contributions	21.8				29.6				39.4			1.1				2			1.9
	Questar Pipeline
	Net periodic cost	$	3.2			$	6.7			$	7.4		$	(0.4	)		$	0.2		$	1.2
	Share of total plan contributions	8.1				11.2				15.1			0.4				0.7			0.6

Operations_by_Line_of_Business1

Operations by Line of Business (Tables)	12 Months Ended
	Dec. 31, 2014
Segment Reporting [Abstract]
Summary of operations by line of business	The following is a summary of operations by line of business for the three years ended December 31, 2014:
		Questar Consol.			Interco. Trans.			Questar Gas			Wexpro			Questar Pipeline			Corp and Other
		(in millions)
	2014
	Revenues
	From unaffiliated customers	$	1,189.30		$	—		$	960.9		$	35.6		$	190.2		$	2.6
	From affiliated companies	—			(424.0		)	—			350.3			73.7			—
	Total Revenues	1,189.30			(424.0		)	960.9			385.9			263.9			2.6
	Operating Expenses
	Cost of sales	186.3			(423.4		)	604.8			—			4			0.9
	Operating and maintenance	189			—			122.5			31.5			34.1			0.9
	General and administrative	127.9			(0.6		)	52.8			32.6			44.1			(1.0		)
	Production and other taxes	66.2			—			17.8			37.6			9.1			1.7
	Depreciation, depletion and amortization	213.7			—			53.6			100.5			54.5			5.1
	Abandonment and impairment	2			—			—			2			—			—
	Total Operating Expenses	785.1			(424.0		)	851.5			204.2			145.8			7.6
	Net gain (loss) from asset sales	1.2			—			0.1			1.6			(0.5		)	—
	Operating Income (Loss)	405.4			—			109.5			183.3			117.6			(5.0		)
	Interest and other income (expense)	6.6			(0.4		)	5.9			1.1			1.2			(1.2		)
	Income from unconsolidated affiliate	3.5			—			—			—			3.5			—
	Interest expense	(63.1		)	0.4			(28.2		)	(0.1		)	(26.1		)	(9.1		)
	Income taxes	(125.9		)	—			(32.0		)	(61.5		)	(35.6		)	3.2
	Net Income (Loss)	$	226.5		$	—		$	55.2		$	122.8		$	60.6		$	(12.1	)
	Identifiable assets	$	4,249.70		$	—		$	1,911.10		$	977.5		$	1,232.40		$	128.7
	Goodwill	9.8			—			5.6			—			4.2			—
	Investment in unconsolidated affiliate	24.7			—			—			—			24.7			—
	Cash capital expenditures, including acquisitions	371.5			—			174.7			114.4			59.4			23
	Accrued capital expenditures, including acquisitions	359.7			—			161.5			114.7			61.3			22.2
	2013
	Revenues
	From unaffiliated customers	$	1,220.00		$	—		$	985.2		$	45.1		$	189.5		$	0.2
	From affiliated companies	—			(372.1		)	0.6			294.8			76.7			—
	Total Revenues	1,220.00			(372.1		)	985.8			339.9			266.2			0.2
	Operating Expenses
	Cost of sales	285.9			(370.9		)	650.6			—			6.1			0.1
	Operating and maintenance	174.3			—			113.1			28.6			32.5			0.1
	General and administrative	121			(0.6		)	52.5			28.7			46.7			(6.3		)
	Production and other taxes	57.4			—			18			28.3			9.3			1.8
	Depreciation, depletion and amortization	194.8			—			49.7			85.8			55.5			3.8
	Asset impairment	80.6			—			—			—			80.6			—
	Other operating expenses	—			(0.6		)	—			0.6			—			—
	Total Operating Expenses	914			(372.1		)	883.9			172			230.7			(0.5		)
	Net loss from asset sales	(0.2		)	—			—			(0.2		)	—			—
	Operating Income	305.8			—			101.9			167.7			35.5			0.7
	Interest and other income (expense)	9.9			(0.7		)	5.1			5			1.8			(1.3		)
	Income from unconsolidated affiliate	3.7			—			—			—			3.7			—
	Interest expense	(56.9		)	0.7			(22.3		)	(0.1		)	(25.8		)	(9.4		)
	Income taxes	(101.3		)	—			(31.9		)	(62.0		)	(7.0		)	(0.4		)
	Net Income (Loss)	$	161.2		$	—		$	52.8		$	110.6		$	8.2		$	(10.4	)
	Identifiable assets	$	4,054.30		$	—		$	1,762.00		$	993.8		$	1,222.80		$	75.7
	Goodwill	9.8			—			5.6			—			4.2			—
	Investment in unconsolidated affiliate	25.6			—			—			—			25.6			—
	Cash capital expenditures, including acquisitions	503.7			—			166.2			249.5			73.4			14.6
	Accrued capital expenditures, including acquisitions	505.6			—			177.3			240.7			70			17.6
	2012
	Revenues
	From unaffiliated customers	$	1,098.90		$	—		$	859.7		$	36.1		$	203.1		$	—
	From affiliated companies	—			(351.0		)	2.5			274.1			74.4			—
	Total Revenues	1,098.90			(351.0		)	862.2			310.2			277.5			—
	Operating Expenses
	Cost of sales	192.3			(347.7		)	533.3			—			6.7			—
	Operating and maintenance	180.8			(0.2		)	119			26.8			35.2			—
	General and administrative	120.8			(0.6		)	51.2			26.8			50			(6.6		)
	Retirement incentive	4.9			—			2.4			0.2			0.9			1.4
Production and other taxes	47.9			—			16.2			20.8			9.1			1.8
Depreciation, depletion and amortization	181.6			—			47.2			77.4			54.3			2.7
Other operating expenses	—			(2.5		)	—			2.5			—			—
Total Operating Expenses	728.3			(351.0		)	769.3			154.5			156.2			(0.7		)
Net gain from asset sales	5.1			—			—			2.4			2.7			—
Operating Income	375.7			—			92.9			158.1			124			0.7
Interest and other income (expense)	7			(0.7		)	5.5			2.8			0.6			(1.2		)
Income from unconsolidated affiliate	3.7			—			—			—			3.7			—
Interest expense	(57.9		)	0.7			(24.1		)	—			(26.3		)	(8.2		)
Income taxes	(116.5		)	—			(27.2		)	(57.0		)	(37.3		)	5
Net Income (Loss)	$	212		$	—		$	47.1		$	103.9		$	64.7		$	(3.7	)
Identifiable assets	$	3,803.30		$	—		$	1,593.50		$	836.1		$	1,298.90		$	74.8
Goodwill	9.8			—			5.6			—			4.2			—
Investment in unconsolidated affiliate	26.5			—			—			—			26.5			—
Cash capital expenditures	370.7			—			162.1			144.5			60.6			3.5
Accrued capital expenditures	366.3			—			161.2			145.5			58.6			1

Wexpro_Acquisitions_of_Produci1

Wexpro Acquisitions of Producing Properties (Tables)	12 Months Ended
	Dec. 31, 2014
Wexpro Acquisition of Producing Properties [Abstract]
Summary information related to the Wexpro acquisitions	Because it contains producing properties, the Canyon Creek acquisition is accounted for under the accounting guidance for business combinations. Summary information related to the Canyon Creek acquisition is shown in the table below. Amounts are provisional and are subject to post-closing adjustments.
		(in millions)
	Fair value of consideration transferred:
	Net cash paid	$	52.4
	Recognized amounts of identifiable assets acquired and liabilities assumed, at fair value:
	Proved properties	$	54.1
	Asset retirement obligations	(1.7		)
	Total fair value of identifiable net assets acquired	$	52.4

Quarterly_Financial_Informatio1

Quarterly Financial Information - Unaudited (Tables)	12 Months Ended
	Dec. 31, 2014
Selected Quarterly Financial Information [Abstract]
Summary of unaudited quarterly financial information	Following is a summary of unaudited quarterly financial information:
		First Quarter		Second Quarter		Third Quarter			Fourth Quarter		Year
		(in millions, except per-share amounts)
	2014
	Revenues	$	456.9	$	201.3	$	157.9		$	373.2	$	1,189.30
	Operating income	148.3		76.5		68.9			111.7		405.4
	Net income	85.1		40.3		38.6			62.5		226.5
	Basic earnings per common share	$	0.48	$	0.23	$	0.22		$	0.36	$	1.29
	Diluted earnings per common share	0.48		0.23		0.22			0.35		1.29
	2013
	Revenues	$	476.9	$	195.6	$	149.6		$	397.9	$	1,220.00
	Operating income (loss)	131.4		71.2		(19.4		)	122.6		305.8
	Net income (loss)	72.9		39.4		(19.2		)	68.1		161.2
	Asset impairment charge, before income taxes	—		—		80.6			—		80.6
	Basic earnings (loss) per common share	$	0.42	$	0.22	$	(0.11	)	$	0.39	$	0.92
	Diluted earnings (loss) per common share	0.41		0.22		(0.11		)	0.39		0.92

Supplemental_Gas_and_Oil_Infor1

Supplemental Gas and Oil Information - Unaudited (Tables)	12 Months Ended
	Dec. 31, 2014
Extractive Industries [Abstract]
Capitalized costs of cost-of-service gas and oil properties net of the related accumulated depreciation and amortization	Capitalized costs of gas and oil properties and the related amounts of accumulated depreciation, depletion and amortization are shown below:
		December 31,
		2014				2013
		(in millions)
	Wexpro
	Proved properties	$	1,675.60			$	1,625.50
	Unproved properties	12.8				0.9
	Total Wexpro capitalized costs	1,688.40				1,626.40
	Accumulated depreciation, depletion and amortization	(757.3		)		(689.8		)
	Net Wexpro capitalized costs	931.1				936.6
	Net Questar Gas capitalized costs	6.4				7.2
	Net capitalized costs	$	937.5			$	943.8
Cost incurred for cost-of-service gas and oil development activities	The costs incurred for gas and oil development activities are displayed in the table below. The costs incurred to develop proved undeveloped reserves were $28.9 million in 2014, $106.3 million in 2013 and $50.4 million in 2012.
		Year Ended December 31,
		2014				2013				2012
		(in millions)
	Property acquisition
	Unproved	$	11.9			$	0.3			$	0.6
	Proved	54.1				106.4				—
	Exploration (capitalized and expensed)	1.6				—				—
	Development	49.1				133.1				146.9
	Total costs incurred	$	116.7			$	239.8			$	147.5
Results of operations for cost-of-service gas- and oil-producing activities, before corporate overhead and interest expenses	Following are the results of operations for gas- and oil-producing activities, excluding corporate overhead and interest costs:
		Year Ended December 31,
		2014				2013				2012
		(in millions)
	Revenues
	From unaffiliated customers	$	35.6			$	45.1			$	36.1
	From affiliated company(1)	350.3				294.8				274.1
	Total revenues	385.9				339.9				310.2
	Production costs	67.5				57.5				50.1
	Exploration expenses	1.6				—				—
	Depreciation, depletion and amortization	100.5				85.8				77.4
	Abandonment and impairment	2				—				—
	Total expenses	171.6				143.3				127.5
	Revenues less expenses	214.3				196.6				182.7
	Income taxes	(71.4		)		(70.5		)		(64.8		)
	Results of operations for gas- and oil-producing activities (excluding corporate overhead and interest costs)	$	142.9			$	126.1			$	117.9
	(1) Primarily represents revenues received from Questar Gas pursuant to the Wexpro agreements. Revenues include reimbursement of general and administrative expenses amounting to $30.5 million in 2014, $27.5 million in 2013 and $26.5 million in 2012.
Estimated quantities of cost-of-service proved gas and oil reserves	Estimated quantities of proved gas and oil reserves are set forth below:
		Natural Gas			Oil and NGL			Natural Gas Equivalents
		(Bcf)			(Mbbl)			(Bcfe)
	Proved Reserves
	Balances at December 31, 2011	758.7			5,344			790.7
	Revisions of previous estimates	(158.4	)		(122	)		(159.0	)
	Extensions and discoveries	154.4			1,612			164
	Production	(57.5	)		(665	)		(61.5	)
	Balances at December 31, 2012	697.2			6,169			734.2
	Revisions of previous estimates	(112.8	)		(1,348	)		(120.8	)
	Extensions and discoveries	153.5			857			158.6
	Purchase of reserves in place	133.9			556			137.2
	Production	(60.6	)		(617	)		(64.3	)
	Balances at December 31, 2013	811.2			5,617			844.9
	Revisions of previous estimates	(220.2	)		(442	)		(222.7	)
	Extensions and discoveries	4			205			5.2
	Purchase of reserves in place	35.9			157			36.8
	Sale of reserves in place	(0.5	)		(219	)		(1.9	)
	Production	(64.3	)		(587	)		(67.8	)
	Balances at December 31, 2014	566.1			4,731			594.5
	Proved Developed Reserves
	Balances at December 31, 2011	491.2			4,420			517.7
	Balances at December 31, 2012	523.9			4,967			553.7
	Balances at December 31, 2013	560			4,384			586.3
	Balances at December 31, 2014	552.9			4,678			581
	Proved Undeveloped Reserves
	Balances at December 31, 2011	267.5			924			273
	Balances at December 31, 2012	173.3			1,202			180.5
	Balances at December 31, 2013	251.2			1,233			258.6
	Balances at December 31, 2014	13.2			53			13.5
Standardized Measure of Discounted Future Cash Flows Relating to Proved Reserves Disclosure [Table Text Block]	The standardized measure of future net cash flows relating to non-cost-of-service proved reserves is presented in the table below:
		December 31,
		2014
		(in millions)
	Future cash inflows	$	190.7
	Future production costs	(65.9		)
	Future income tax expenses	(43.7		)
	Future net cash flows	81.1
	10% annual discount for estimated timing of net cash flows	(34.4		)
	Standardized measure of discounted future net cash flows	$	46.7
Schedule of Changes in Standardized Measure of Discounted Future Net Cash Flows [Table Text Block]	The principal sources of change in the standardized measure of future net cash flows relating to non-cost-of-service proved reserves are presented in the table below:
		Year Ended December 31,
		2014
		(in millions)
	Balance at beginning of year	$	—
	Net increase due to purchases of reserves in place	46.8
	Sales of gas and oil produced during the period, net of production costs	(1.5		)
	Net change in prices and production costs related to future production	0.8
	Accretion of discount	0.6
	Net Change	46.7
	Balance at end of year	$	46.7

Summary_of_Significant_Account3

Summary of Significant Accounting Policies (Details) (USD $)	3 Months Ended				12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
					bbl
					lines_of_business
Nature of Business [Abstract]
Number Of Business Lines					3
Revenue Recognition [Abstract]
Percentage of operating income retained by Wexpro after expenses (in hundredths)					46.00%
Cost of Sales [Abstract]
Other cost of sales					$900,000	$100,000	$0
Total cost of sales					186,300,000	285,900,000	192,300,000
Cash and Cash Equivalents [Abstract]
Cash equivalent maturity maximum (months)					3 months
Depreciation, depletion and amortization [Abstract]
Barrel of oil conversion to 6,000 cubic feet of natural gas					1
Impairment of Long-Lived Assets [Abstract]
Abandonment and impairment	0	80,600,000	0	0	2,000,000	80,600,000	0
Capitalized Interest and Allowance for Funds Used During Construction [Abstract]
AFUDC (recorded as an increase in interest and other income)					2,800,000	6,300,000	2,300,000
Capitalized interest costs (recorded as a reduction of interest expense)					1,000,000	800,000	200,000
Credit Risk [Abstract]
Bad debt expense					1,700,000	200,000	1,200,000
Allowance for bad debts	1,700,000				1,700,000	1,700,000
Income Taxes [Abstract]
Unrecognized tax benefits	0				0	0	0
Percent of capital expenditures deductible for federal income tax purposes under current statutes (in hundredths)					50.00%	50.00%	50.00%
Estimated net operating loss (NOL) carryforward							26,900,000
Questar Gas [Member]
Revenue Recognition [Abstract]
Rate Adjustment Time Period					6 months
Deferred CET adjustments amortization period					12 months
Rate adjustment limitation as a percent of non-gas revenues (In hundredths)					5.00%
Cost of Sales [Abstract]
Gas purchases					136,500,000	186,600,000	104,200,000
Operator service fee					349,700,000	294,600,000	274,000,000
Transportation and storage					79,600,000	80,100,000	79,600,000
Gathering					21,000,000	18,800,000	20,500,000
Royalties					60,100,000	44,300,000	32,000,000
Storage (injection) withdrawal, net					-1,100,000	-800,000	1,900,000
Purchased-gas account adjustment					-45,800,000	22,000,000	16,100,000
Other					4,800,000	5,000,000	5,000,000
Total cost of natural gas sold (excluding operating expenses shown separately) / Total Questar Gas cost of natural gas sold					604,800,000	650,600,000	533,300,000
Elimination of Questar Gas cost of natural gas sold - affiliated companies					-423,400,000	-370,900,000	-347,700,000
Total Questar Gas cost of natural gas sold - unaffiliated parties					181,400,000	279,700,000	185,600,000
Contributions in aid of construction [Abstract]
Time period contributions are refundable if other customers pay to connect to the line (in years)					5 years
Time period if no other customers connect to the line the funds are not refundable (in years)					5 years
Depreciation, depletion and amortization [Abstract]
Questar Gas distribution plant					2.70%	2.70%	2.80%
Capitalized Interest and Allowance for Funds Used During Construction [Abstract]
AFUDC (recorded as an increase in interest and other income)					900,000	0	0
Capitalized interest costs (recorded as a reduction of interest expense)					500,000	200,000	100,000
Credit Risk [Abstract]
Time period in months uncollected accounts are written off					6 months
Wexpro [Member]
Depreciation, depletion and amortization [Abstract]
Cost-of-service gas and oil properties, per Mcfe					1.75	1.56	1.49
Impairment of Long-Lived Assets [Abstract]
Abandonment and impairment					2,000,000
Capitalized Interest and Allowance for Funds Used During Construction [Abstract]
AFUDC (recorded as an increase in interest and other income)					800,000	4,600,000	2,100,000
Questar Pipeline [Member]
Investment in Unconsolidated Affiliate [Abstract]
Ownership percentage in unconsolidated affiliate (in hundredths)					50.00%
Cost of Sales [Abstract]
Total Questar Pipeline cost of sales					4,000,000	6,100,000	6,700,000
Depreciation, depletion and amortization [Abstract]
Questar Pipeline transportation, storage and other energy services					3.20%	3.40%	3.40%
Impairment of Long-Lived Assets [Abstract]
Abandonment and impairment					0	80,600,000	0
Capitalized Interest and Allowance for Funds Used During Construction [Abstract]
AFUDC (recorded as an increase in interest and other income)					1,100,000	1,700,000	200,000
Capitalized interest costs (recorded as a reduction of interest expense)					$500,000	$600,000	$100,000

Earnings_Per_Share_Details

Earnings Per Share (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reconciliation of the components of basic and diluted shares in the EPS Calculation [Abstract]
Weighted-average basic common shares outstanding	175,800,000	175,400,000	176,500,000
Potential number of shares issuable under the Company's LTSIP	300,000	600,000	1,000,000
Weighted-average diluted common shares outstanding	176,100,000	176,000,000	177,500,000
Dividend Reinvestment and Stock Purchase Plan [Abstract]
Shares issued	0	0	0
Shares reserved for future issuance	19,996,038

Accumulated_Other_Comprehensiv2

Accumulated Other Comprehensive Income (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balances	($141.80)		($246.30)
OCI before reclassifications	-113.6		135.7
Reclassified from AOCI	17.2	[1]	33.5	[1]
Income taxes
Attributable to OCI before reclassifications	43.4		-51.8
Attributable to amounts reclassified from AOCI	-6.5	[2]	-12.9	[2]
Total income taxes	36.9		-64.7		12.3
Net other comprehensive income (loss)	-59.5		104.5		-20.2
Ending balances	-201.3		-141.8		-246.3
Reclassification Adjustment out of Accumulated Other Comphrensive Income [Abstract]
Income tax (expense) benefit	-125.9		-101.3		-116.5
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Change in unrecognized actuarial loss					13.6
Change in unrecognized prior service cost					-1
Interest rate cash flow hedges					-0.2
Change in fair value of long-term investment					-0.1
Total income taxes	36.9		-64.7		12.3
Pension Plans, Defined Benefit [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balances	-108.8		-203.5
OCI before reclassifications	-111.5		123.4
Reclassified from AOCI	16	[1]	30	[1]
Income taxes
Attributable to OCI before reclassifications	42.6		-47.2
Attributable to amounts reclassified from AOCI	-6.1	[2]	-11.5	[2]
Total income taxes	36.5		-58.7
Net other comprehensive income (loss)	-59		94.7
Ending balances	-167.8		-108.8
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Total income taxes	36.5		-58.7
Actuarial Loss [Member] | Pension Plans, Defined Benefit [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balances	-108.4		-202.4
OCI before reclassifications	-111.5		123.4
Reclassified from AOCI	15.4	[1]	28.9	[1]
Income taxes
Attributable to OCI before reclassifications	42.6		-47.2
Attributable to amounts reclassified from AOCI	-5.9	[2]	-11.1	[2]
Total income taxes	36.7		-58.3
Net other comprehensive income (loss)	-59.4		94
Ending balances	-167.8		-108.4
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Total income taxes	36.7		-58.3
Actuarial Loss [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balances	-10.2		-19.7
OCI before reclassifications	-2.1		12.4
Reclassified from AOCI	0.7	[1]	3	[1]
Income taxes
Attributable to OCI before reclassifications	0.8		-4.7
Attributable to amounts reclassified from AOCI	-0.3	[2]	-1.2	[2]
Total income taxes	0.5		-5.9
Net other comprehensive income (loss)	-0.9		9.5
Ending balances	-11.1		-10.2
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Total income taxes	0.5		-5.9
Prior service cost [Member] | Pension Plans, Defined Benefit [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balances	-0.4		-1.1
OCI before reclassifications	0		0
Reclassified from AOCI	0.6	[1]	1.1	[1]
Income taxes
Attributable to OCI before reclassifications	0		0
Attributable to amounts reclassified from AOCI	-0.2	[2]	-0.4	[2]
Total income taxes	-0.2		-0.4
Net other comprehensive income (loss)	0.4		0.7
Ending balances	0		-0.4
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Total income taxes	-0.2		-0.4
Long-term investment [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balances	0.1		0.1
OCI before reclassifications	0		-0.1
Reclassified from AOCI	0	[1]	0	[1]
Income taxes
Attributable to OCI before reclassifications	0		0.1
Attributable to amounts reclassified from AOCI	0	[2]	0	[2]
Total income taxes	0		0.1
Net other comprehensive income (loss)	0		0
Ending balances	0.1		0.1
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Total income taxes	0		0.1
Questar Pipeline [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balances	-22.9
Income taxes
Total income taxes	-0.1		-0.2		-0.2
Net other comprehensive income (loss)	0.4		0.3		0.2
Ending balances	-22.5		-22.9
Reclassification Adjustment out of Accumulated Other Comphrensive Income [Abstract]
Income tax (expense) benefit	-35.6		-7		-37.3
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Total income taxes	-0.1		-0.2		-0.2
Questar Pipeline [Member] | Interest rate cash flow hedges [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balances	-22.9		-23.2
OCI before reclassifications	0		0
Reclassified from AOCI	0.5	[1]	0.5	[1]
Income taxes
Attributable to OCI before reclassifications	0		0
Attributable to amounts reclassified from AOCI	-0.1	[2]	-0.2	[2]
Total income taxes	-0.1		-0.2
Net other comprehensive income (loss)	0.4		0.3
Ending balances	-22.5		-22.9
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Total income taxes	-0.1		-0.2
Questar Pipeline [Member] | Interest rate cash flow hedges [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comphrensive Income [Abstract]
Interest income (expense)	-0.5		-0.5
Income tax (expense) benefit	$0.10		$0.20
[1]	Interest rate cash flow hedge amounts are included in their entirety as charges to interest expense on the Consolidated Statements of Income.
[2]	Income tax reclassifications related to interest rate cash flow hedge amounts are included in their entirety as credits to income taxes on the Consolidated Statements of Income.

Asset_Retirement_Obligations_D

Asset Retirement Obligations (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
AROs at beginning of year	$67.70	$67.20
Accretion	3.4	3.3
Liabilities incurred	3.3	6
Revisions in estimated cash flows	-2.7	-6.9
Liabilities settled	-2.4	-1.9
AROs at end of year	69.3	67.7
Asset Retirement Obligations By Line Of Business [Abstract]
AROs by line of business	69.3	67.7
Questar Gas [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
AROs at end of year	0.6	0.6
Asset Retirement Obligations By Line Of Business [Abstract]
AROs by line of business	0.6	0.6
Wexpro [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
AROs at end of year	66.3	64.9
Asset Retirement Obligations By Line Of Business [Abstract]
AROs by line of business	66.3	64.9
Questar Pipeline [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
AROs at end of year	2.3	2.2
Asset Retirement Obligations By Line Of Business [Abstract]
AROs by line of business	2.3	2.2
Questar Corp [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
AROs at end of year	0.1	0
Asset Retirement Obligations By Line Of Business [Abstract]
AROs by line of business	$0.10	$0

Fair_Value_Measurements_Detail

Fair Value Measurements (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Jun. 30, 2014	Sep. 30, 2013
In Millions, unless otherwise specified
Carrying Amount [Member]
Financial assets [Abstract]
Cash and cash equivalents	$32	$16
Long-term investment	15.7	17.3
Financial liabilities [Abstract]
Short-term debt	347	276
Long-term debt, including current portion	1,245.20	1,247.10
Estimated Fair Value [Member] | Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	32	16
Financial liabilities [Abstract]
Short-term debt	347	276
Estimated Fair Value [Member] | Level 2 [Member]
Financial liabilities [Abstract]
Long-term debt, including current portion	1,356.10	1,310.80
Estimated Fair Value [Member] | Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Financial assets [Abstract]
Long-term investment	15.7	17.3
Questar Gas [Member] | Carrying Amount [Member]
Financial assets [Abstract]
Cash and cash equivalents	19.8	8.8
Financial liabilities [Abstract]
Notes payable to Questar	119.3	17.7
Long-term debt, including current portion	534.5	534.5
Questar Gas [Member] | Estimated Fair Value [Member] | Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	19.8	8.8
Financial liabilities [Abstract]
Notes payable to Questar	119.3	17.7
Questar Gas [Member] | Estimated Fair Value [Member] | Level 2 [Member]
Financial liabilities [Abstract]
Long-term debt, including current portion	607.2	568
Wexpro [Member] | Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 3 [Member]
Financial liabilities [Abstract]
Property, Plant, and Equipment, Fair Value Disclosure			0
Questar Pipeline [Member] | Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 3 [Member]
Financial liabilities [Abstract]
Property, Plant, and Equipment, Fair Value Disclosure				0
Questar Pipeline [Member] | Carrying Amount [Member]
Financial assets [Abstract]
Cash and cash equivalents	7.4	2.7
Notes receivable from Questar	40.1	29.4
Financial liabilities [Abstract]
Long-term debt, including current portion	458.8	458.9
Questar Pipeline [Member] | Estimated Fair Value [Member] | Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	7.4	2.7
Notes receivable from Questar	40.1	29.4
Questar Pipeline [Member] | Estimated Fair Value [Member] | Level 2 [Member]
Financial liabilities [Abstract]
Long-term debt, including current portion	$495.50	$486

Derivatives_and_Hedging_Detail

Derivatives and Hedging (Details) (USD $)	3 Months Ended	12 Months Ended		3 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2014	Dec. 31, 2011	Mar. 31, 2012	Dec. 31, 2013	Jun. 30, 2011
Derivative [Line Items]
Derivative assets or liabilities outstanding		$0			$0
Cash Flow Hedging [Member] | Questar Pipeline [Member] | Forward Starting Interest Rate Swaps Terminated Q4 2011 [Member]
Derivative [Line Items]
Notional amount	150		150
Debt instrument, face amount	180		180
Derivative, loss on derivative	37.3
Life of long-term debt associated with cash flow hedges			30 years
Date through which reclassifications into earnings from AOCI will take place		2041
Pre-tax net losses expected to be reclassified from AOCI to earnings		0.6
Reclassification from accumulated OCI to income, estimated time to transfer		12 months
Fair Value Hedging [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Notional amount						125
Weighted-average fixed interest rate (in hundredths)						2.75%
Derivative, gain on derivative				$7.20

Debt_Shortterm_Debt_Details

Debt Short-term Debt (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Short-term Debt [Line Items]
Commercial paper with various interest rates	$347	$276
Revolving credit arrangement [Member]
Short-term Debt [Line Items]
Revolving credit facility credit commitments	750	750	500
Revolving credit facility, maturity date	19-Apr-18	19-Apr-18	31-Aug-16
Consolidated funded debt percentage of consolidated capital permitted under credit facility amendment	70.00%
Commercial paper with various interest rates	0
Revolving credit facility, remaining borrowing capacity	403
Commercial paper [Member]
Short-term Debt [Line Items]
Commercial paper with various interest rates	347	276
Weighted-average interest rate at end of year	0.23%	0.27%
Questar Gas [Member]
Short-term Debt [Line Items]
Notes payable to parent	119.3	17.7
Questar Gas [Member] | Notes payable to Questar [Member]
Short-term Debt [Line Items]
Weighted-average interest rate at end of year	0.25%	0.30%
Notes payable to parent	119.3	17.7
Questar Pipeline [Member] | Notes payable to Questar [Member]
Short-term Debt [Line Items]
Notes payable to parent	$0	$0

Debt_Longterm_Debt_Details

Debt Long-term Debt (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Total long-term debt outstanding	$1,244.60	$1,244.60
Less current portion	-25.1	0
Less unamortized debt discount	-1.9	-2.1
Plus unamortized debt premium	0.5	0.7
Plus fair value hedge adjustment	2	3.9
Total long-term debt, less current portion	1,220.10	1,247.10
Questar Corp [Member] | 2.75% Notes due 2016 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	2.75%
Debt Instrument, Maturity Date	1-Feb-16
Total long-term debt outstanding	250	250
Questar Gas [Member]
Debt Instrument [Line Items]
Total long-term debt outstanding	534.5	534.5
Total long-term debt, less current portion	534.5	534.5
Questar Gas [Member] | 5.31% and 6.85% Medium-term Notes due 2017 and 2018 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum	5.31%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum	6.85%
Debt Instrument, Maturity Date Range, Start	10-Oct-17
Debt Instrument, Maturity Date Range, End	15-Mar-18
Total long-term debt outstanding	84.5	84.5
Questar Gas [Member] | 6.30% Notes due 2018 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.30%
Debt Instrument, Maturity Date	1-Apr-18
Total long-term debt outstanding	50	50
Questar Gas [Member] | 2.98% Notes due 2024 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	2.98%
Debt Instrument, Maturity Date	1-Dec-24
Total long-term debt outstanding	40	40
Questar Gas [Member] | 3.28% Notes due 2027 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.28%
Debt Instrument, Maturity Date	1-Dec-27
Total long-term debt outstanding	110	110
Questar Gas [Member] | 7.20% Notes due 2038 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.20%
Debt Instrument, Maturity Date	1-Apr-38
Total long-term debt outstanding	100	100
Questar Gas [Member] | 4.78% Notes due 2043 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.78%	4.78%
Debt Instrument, Maturity Date	1-Dec-43
Debt instrument, face amount		90
Debt Instrument, Term		30 years
Total long-term debt outstanding	90	90
Questar Gas [Member] | 4.83% Notes due 2048 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.83%	4.83%
Debt Instrument, Maturity Date	1-Dec-48
Debt instrument, face amount		60
Debt Instrument, Term		35 years
Total long-term debt outstanding	60	60
Questar Gas [Member] | 4.78% and 4.83% Notes due 2043 and 2048 [Member]
Debt Instrument [Line Items]
Net Proceeds from Issuance of Long-Term Debt		149
Questar Pipeline [Member]
Debt Instrument [Line Items]
Total long-term debt outstanding	460.1	460.1
Total long-term debt, less current portion	433.7	458.9
Questar Pipeline [Member] | 6.45% and 6.48% Medium-term Notes due 2015 and 2018 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum	6.45%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum	6.48%
Debt Instrument, Maturity Date Range, Start	15-Oct-15
Debt Instrument, Maturity Date Range, End	11-Dec-18
Total long-term debt outstanding	30.1	30.1
Questar Pipeline [Member] | 5.83% Notes due 2018 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.83%
Debt Instrument, Maturity Date	1-Feb-18
Total long-term debt outstanding	250	250
Questar Pipeline [Member] | 4.875% Notes due 2041 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.88%
Debt Instrument, Maturity Date	1-Dec-41
Total long-term debt outstanding	$180	$180

Debt_Maturities_of_Longterm_De

Debt Maturities of Long-term Debt (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014
Debt Maturities [Line Items]
Number of years of long-term debt maturities	5 years
Maturities of Long-term Debt [Abstract]
2015	$25.10
2016	250
2017	14.5
2018	375
2019	0
Questar Corp [Member]
Maturities of Long-term Debt [Abstract]
2015	0
2016	250
2017	0
2018	0
2019	0
Questar Gas [Member]
Maturities of Long-term Debt [Abstract]
2015	0
2016	0
2017	14.5
2018	120
2019	0
Questar Pipeline [Member]
Maturities of Long-term Debt [Abstract]
2015	25.1
2016	0
2017	0
2018	255
2019	$0

Income_Taxes_Details

Income Taxes (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Federal [Abstract]
Current	$69.60	$66.50	$1.30
Deferred	47.5	18.4	112
State [Abstract]
Current	6.9	10.7	0
Deferred	2.1	5.9	3.6
Deferred investment tax credits recognized	-0.2	-0.2	-0.4
Total income tax expense	125.9	101.3	116.5
Difference between the statutory federal income tax rate and the Company's effective income tax rate [Abstract]
Federal income taxes statutory rate (in hundredths)	35.00%	35.00%	35.00%
State income taxes, net of federal income tax benefit (in hundredths)	1.60%	4.00%	1.00%
Tax benefits from dividends paid to employee stock plan (in hundredths)	-1.00%	-1.00%	-0.50%
Other (in hundredths)	0.10%	0.60%	0.00%
Effective income tax rate (in hundredths)	35.70%	38.60%	35.50%
Deferred tax liabilities [Abstract]
Property, plant and equipment	827	797.5
Employee benefits	52	30.2
Other	0.5	4.9
Deferred tax liabilities - noncurrent	879.5	832.6
Deferred tax assets [Abstract]
Asset retirement obligations	22.5	22.7
Pension and other postretirement benefits	110.8	73.7
Deferred compensation	15.7	14.5
Hedging activities	13.2	13.4
State tax credits	3.3	3.4
Valuation allowance	-1.6	-2.3
Deferred tax assets, net of allowance - noncurrent	163.9	125.4
Net deferred income tax liability - noncurrent	715.6	707.2
Deferred income taxes - current [Abstract]
Deferred tax assets - current	15.7	10.1
Deferred tax liabilities - current	9.9	0.4
Net deferred income tax asset - current	5.8	9.7
Questar Gas [Member]
Federal [Abstract]
Current	-11.9	6	-18.7
Deferred	42.4	23.5	45.1
State [Abstract]
Current	-1.9	0.6	0
Deferred	3.6	2	1.2
Deferred investment tax credits recognized	-0.2	-0.2	-0.4
Total income tax expense	32	31.9	27.2
Difference between the statutory federal income tax rate and the Company's effective income tax rate [Abstract]
Federal income taxes statutory rate (in hundredths)	35.00%	35.00%	35.00%
State income taxes, net of federal income tax benefit (in hundredths)	1.30%	2.10%	1.00%
Amortization of investment tax credits related to rate-regulated assets (in hundredths)	-0.20%	-0.30%	-0.40%
Other (in hundredths)	0.60%	0.90%	1.00%
Effective income tax rate (in hundredths)	36.70%	37.70%	36.60%
Deferred tax liabilities [Abstract]
Property, plant and equipment	354.4	322.6
Employee benefits	23.5	14
Other	0.5	4.9
Deferred tax liabilities - noncurrent	378.4	341.5
Deferred tax assets [Abstract]
Deferred compensation	0.9	0.8
Deferred tax assets, net of allowance - noncurrent	0.9	0.8
Net deferred income tax liability - noncurrent	377.5	340.7
Deferred income taxes - current [Abstract]
Deferred tax assets - current	3.6	3.3
Deferred tax liabilities - current	9.9	0.4
Net deferred income tax asset - current	0	2.9
Net deferred income tax (liability) - current	-6.3	0
Questar Pipeline [Member]
Federal [Abstract]
Current	21.9	20.2	-5.4
Deferred	11.6	-15.2	40.4
State [Abstract]
Current	1	1.3	1.6
Deferred	1.1	0.7	0.7
Total income tax expense	35.6	7	37.3
Difference between the statutory federal income tax rate and the Company's effective income tax rate [Abstract]
Federal income taxes statutory rate (in hundredths)	35.00%	35.00%	35.00%
State income taxes, net of federal income tax benefit (in hundredths)	1.50%	8.50%	1.60%
Other (in hundredths)	0.50%	2.60%	0.00%
Effective income tax rate (in hundredths)	37.00%	46.10%	36.60%
Deferred tax liabilities [Abstract]
Property, plant and equipment	246.1	237.1
Employee benefits	10.8	7.4
Deferred tax liabilities - noncurrent	256.9	244.5
Deferred tax assets [Abstract]
Deferred compensation	2	1.8
Hedging activities	13.2	13.4
State tax credits	0.7	0.7
Valuation allowance	-0.4	0
Deferred tax assets, net of allowance - noncurrent	15.5	15.9
Net deferred income tax liability - noncurrent	241.4	228.6
Deferred income taxes - current [Abstract]
Net deferred income tax asset - current	$1.90	$1.80

Contingencies_Commitments_and_2

Contingencies, Commitments and Leases Contingencies (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014
Loss Contingencies [Line Items]
Environmental expense for previously-owned chemical company	$5
Accrual for environmental loss contingency on previously-owned chemical company	5.4
Wexpro [Member]
Loss Contingencies [Line Items]
Overriding royalty interest demanded in Rocky Mountain Resources litigation (in hundredths)	4.00%
Maximum expected loss on Rocky Mountain Resources litigation	14.1
Unaffiliated Entity [Member]
Loss Contingencies [Line Items]
Maximum expected loss on Rocky Mountain Resources litigation	$16.20

Contingencies_Commitments_and_3

Contingencies, Commitments and Leases Commitments (Details) (Questar Gas [Member], USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Long-term Purchase Commitment [Line Items]
Natural gas purchased under agreements	$135.80	$186.50	$104.10
Gas purchase agreements [Member]
Long-term Purchase Commitment [Line Items]
2015	18.3
2016	15.2
2017	18.2
2018	21.2
2019	21.2
Transportation and storage commitments - Related party [Member]
Long-term Purchase Commitment [Line Items]
2015	74.4
2016	73.4
2017	46.7
2018	13.4
2019	5.2
Transportation and gathering commitments - Third party [Member]
Long-term Purchase Commitment [Line Items]
2015	29
2016	29
2017	29
2018	26.3
2019	$24.90

Contingencies_Commitments_and_4

Contingencies, Commitments and Leases Leases (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2012	Dec. 31, 2013
Leases, Operating [Abstract]
Expiration date of lease on former headquarters building		30-Apr-12
Rental expense		$1.50
Leases, Capital [Abstract]
New headquarters building lease term	17 years
Commencement date of capital lease on new headquarters building	1-May-12
Description of Lessee Leasing Arrangements, Capital Leases	Rental payments under the lease escalate at a rate of 3% per year during the lease term. The lease agreement does not include bargain renewal periods or material rent holidays and is not subject to contingent rent or other unusual provisions.
Assets under capital lease included in other property, plant and equipment	40.8		40.8
Assets under capital lease included in other accumulated depreciation, depletion and amortization	6.7		4.3
Current capital lease obligation	1		0.9
Noncurrent capital lease obligation	37.4		38.4
Future minimum payments and present value of net minimum payments under capital lease [Abstract]
2015	3.1
2016	3.5
2017	3.6
2018	3.7
2019	3.8
After 2019	41
Total minimum lease payments	58.7
Less: amount representing interest	-20.3
Present value of minimum lease payments at year-end	$38.40

Wexpro_and_Wexpro_II_Agreement2

Wexpro and Wexpro II Agreements (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Agreement [Line Items]
Percentage of Wexpro retention (in hundredths)	46.00%
Wexpro net investment base at December 31, (in millions)	$649	$589.70	$531.10
Average annual rate of return (after tax) (in hundredths)	17.90%	19.70%	19.90%
Oil and NGL income sharing (in millions)	$0	$0.60	$2.50
Trail acquisition specified Questar Gas supply percentage (in hundredths)	65.00%
Gas-development drilling [Member]
Agreement [Line Items]
Percentage of Wexpro after-tax rate of return (in hundredths)	20.00%
Percentage of premium added to base after-tax rate of return (in hundredths)	8.00%
Natural gas operations [Member]
Agreement [Line Items]
Percentage of Wexpro after-tax rate of return (in hundredths)	12.00%
Developmental oil drilling [Member]
Agreement [Line Items]
Percentage of Wexpro after-tax rate of return (in hundredths)	17.00%
Percentage of premium added to base after-tax rate of return (in hundredths)	5.00%
Percentage of Wexpro retention (in hundredths)	46.00%
Percentage of Questar Gas retention (in hundredths)	54.00%
Crude oil production [Member]
Agreement [Line Items]
Percentage of Wexpro after-tax rate of return (in hundredths)	12.00%
Percentage of Wexpro retention (in hundredths)	46.00%
Acquired natural gas production [Member]
Agreement [Line Items]
Percentage of Wexpro after-tax rate of return (in hundredths)	7.60%

Rate_Regulation_Details

Rate Regulation (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	$79.60	$35.80
Noncurrent regulatory assets	25	20.5
Current regulatory liabilities	13.4	14.1
Noncurrent regulatory liabilities	69.9	61.2
Questar Gas [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	78.3	30.2
Noncurrent regulatory assets	21.3	16.1
Current regulatory liabilities	12.5	11.1
Noncurrent regulatory liabilities	60.9	53
Rate Changes [Abstract]
Current authorized return on equity in Utah	9.85%
Amount of increase in annual revenues in Utah	7.6
Amount of increase in customer rates in Wyoming	1.5
Current authorized return on equity in Wyoming	9.16%
Future Authorized Return on Equity in Wyoming	9.50%
Questar Gas [Member] | Purchased-gas adjustment [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	39.2	0
Noncurrent regulatory assets	0	0
Current regulatory liabilities	0	7.4
Noncurrent regulatory liabilities	0	0
Questar Gas [Member] | Demand-side management [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	0	11.2
Noncurrent regulatory assets	0	0
Current regulatory liabilities	0.3	0
Noncurrent regulatory liabilities	0	0
Questar Gas [Member] | Contract withholding [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	13.6	8.1
Noncurrent regulatory assets	0	0
Questar Gas [Member] | Deferred cost-of-service gas charges [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	25.5	10.9
Noncurrent regulatory assets	9.3	2
Regulatory asset remaining recovery period (in years)	12 months
Regulatory asset amortization period (in years)	18-year
Questar Gas [Member] | Cost of reacquired debt [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	0	0
Noncurrent regulatory assets	4.3	4.8
Regulatory asset remaining recovery period (in years)	8 years
Questar Gas [Member] | Pipeline integrity costs [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	0	0
Noncurrent regulatory assets	7.7	9.3
Amount of allowed yearly recovery through pipeline integrity regulations	7
Questar Gas [Member] | CET [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory liabilities	12.1	3.3
Noncurrent regulatory liabilities	0	0
Questar Gas [Member] | Cost of plant removal [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory liabilities	0	0
Noncurrent regulatory liabilities	60.7	52.7
Questar Gas [Member] | Income taxes recoverable from/refundable to customers [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory liabilities	0	0
Noncurrent regulatory liabilities	0.2	0.3
Questar Gas [Member] | Other Regulatory Assets (Liabilities) [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory liabilities	0.1	0.4
Noncurrent regulatory liabilities	0	0
Questar Pipeline [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	1.3	5.6
Noncurrent regulatory assets	3.7	4.4
Current regulatory liabilities	0.9	3
Noncurrent regulatory liabilities	9	8.2
Questar Pipeline [Member] | Cost of reacquired debt [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	0	0
Noncurrent regulatory assets	2.2	2.6
Regulatory asset remaining recovery period (in years)	6 years
Questar Pipeline [Member] | Gas imbalance [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	1.2	5.3
Noncurrent regulatory assets	0	0
Current regulatory liabilities	0.8	3
Noncurrent regulatory liabilities	0	0
Questar Pipeline [Member] | Revenue sharing [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	0.1	0.3
Noncurrent regulatory assets	0	0
Current regulatory liabilities	0.1	0
Noncurrent regulatory liabilities	0	0
Questar Pipeline [Member] | Income taxes recoverable from/refundable to customers [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	0	0
Noncurrent regulatory assets	0.3	0.6
Questar Pipeline [Member] | Postretirement medical [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory liabilities	0	0
Noncurrent regulatory liabilities	9	8.2
Questar Pipeline [Member] | Other Regulatory Assets (Liabilities) [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	0	0
Noncurrent regulatory assets	$1.20	$1.20

ShareBased_Compensation_Genera

Share-Based Compensation General (Details) (USD $)	12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	$12.40	$10.20	$9.90
Cash flow from income tax benefits in excess of recognized compensation expense	2.2	14	0
Shares available for future grant (in shares)	4,053,586
Total unrecognized compensation cost related to all share-based award types	8.8
Weighted-average period of recognition for total unrecognized compensation cost (in months)	12 months
Questar Gas [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	1.6	1.4	1.2
Questar Pipeline [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	$2.10	$2	$2

ShareBased_Compensation_Stock_

Share-Based Compensation Stock options (Detail) (Stock Options [Member], USD $)	12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2014	Dec. 31, 2013
	installments
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Stock option minimum term (in years)	5 years
Stock option maximum term (in years)	10 years
Stock option majority minimum term (in years)	7 years
Stock option majority maximum term (in years)	10 years
Minimum number of installments for vesting of options held by employees	3
Maximum number of installments for vesting of options held by employees	4
Number of unvested options at end of period	0		0
Stock option transactions under the terms of the LTSIP - shares [Roll Forward]
Balance at December 31, 2013	717,840
Granted	0
Exercised	-282,333
Forfeited	0
Balance at December 31, 2014	435,507
Stock option transactions under the terms of the LTSIP - exercise price [Roll Forward]
Balance at December 31, 2013	$11.84
Exercised	$10.88
Balance at December 31, 2014	$12.46
Stock options outstanding, exercisable and unvested at end of period, by exercise price range [Abstract]
Number of outstanding options at end of period (in shares)		435,507
Weighted-average remaining term (in years and months)	1 year 6 months
Weighted-average exercise price (in dollars per share)	$12.46
Number of exercisable options at end of period (in shares)	435,507
Weighted-average exercise price, options exercisable (in dollars per share)	$12.46
Stock options, additional disclosures [Abstract]
Aggregate intrinsic value of outstanding options (in shares)	$5.60
Aggregate intrinsic value of exercisable options	$5.60
Number of Questar stock options held by certain officers, employees and nonemployee directors of QEP (in shares)	245,332
Weighted-average exercise price of Questar stock options awarded to certain officers, employees and nonemployee directors of QEP (in dollars per share)	$11.34
Weighted-average remaining life of Questar stock options held by certain officers, employees and nonemployee directors of QEP (in years)	1 year 5 months
Options Outstanding Exercise Price Range 1 [Member]
Stock options outstanding, exercisable and unvested at end of period, by exercise price range [Abstract]
Minimum exercise price of options outstanding (in dollars per share)	$11.40
Maximum exercise price of options outstanding (in dollars per share)	$11.40
Number of outstanding options at end of period (in shares)		239,174
Weighted-average remaining term (in years and months)	1 year 2 months
Weighted-average exercise price (in dollars per share)	$11.40
Number of exercisable options at end of period (in shares)	239,174
Weighted-average exercise price, options exercisable (in dollars per share)	$11.40
Options Outstanding Exercise Price Range 2 [Member]
Stock options outstanding, exercisable and unvested at end of period, by exercise price range [Abstract]
Minimum exercise price of options outstanding (in dollars per share)	$13.10
Maximum exercise price of options outstanding (in dollars per share)	$17.35
Number of outstanding options at end of period (in shares)		196,333
Weighted-average remaining term (in years and months)	2 years 0 months
Weighted-average exercise price (in dollars per share)	$13.75
Number of exercisable options at end of period (in shares)	196,333
Weighted-average exercise price, options exercisable (in dollars per share)	$13.75
Minimum [Member]
Stock option transactions under the terms of the LTSIP - exercise price [Roll Forward]
Balance at December 31, 2013	$7.84
Exercised	$7.84
Balance at December 31, 2014	$11.40
Maximum [Member]
Stock option transactions under the terms of the LTSIP - exercise price [Roll Forward]
Balance at December 31, 2013	$17.35
Exercised	$13.24
Balance at December 31, 2014	$17.35
Questar Gas [Member]
Stock options outstanding, exercisable and unvested at end of period, by exercise price range [Abstract]
Number of outstanding options at end of period (in shares)		0
Questar Pipeline [Member]
Stock options outstanding, exercisable and unvested at end of period, by exercise price range [Abstract]
Minimum exercise price of options outstanding (in dollars per share)	$11.40
Maximum exercise price of options outstanding (in dollars per share)	$13.10
Number of outstanding options at end of period (in shares)		58,933
Weighted-average remaining term (in years and months)	1 year 6 months
Weighted-average exercise price (in dollars per share)	$11.95
Number of exercisable options at end of period (in shares)	58,933
Weighted-average exercise price, options exercisable (in dollars per share)	$11.95

ShareBased_Compensation_Restri

Share-Based Compensation Restricted shares (Details) (Restricted Shares [Member], USD $)	12 Months Ended
	Dec. 31, 2014
Transactions under the terms of the LTSIP - shares [Roll Forward]
Balance at December 31, 2013	392,327
Granted	0
Vested	-289,312
Forfeited	-661
Balance at December 31, 2014	102,354
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2013	$18.28
Vested	$17.88
Forfeited	$20
Balance at December 31, 2014	$19.41
Share-based awards other than options, additional disclosures [Abstract]
Vesting period for share-based award (in years)	3 years
Weighted-average remaining vesting period of share-based awards issued (in months)	2 months
Minimum [Member]
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2013	$15.22
Vested	$15.22
Forfeited	$19.39
Balance at December 31, 2014	$17.97
Maximum [Member]
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2013	$21.53
Vested	$21.53
Forfeited	$20.90
Balance at December 31, 2014	$21.53
Questar Gas [Member]
Transactions under the terms of the LTSIP - shares [Roll Forward]
Balance at December 31, 2014	12,635
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2014	$19.39
Share-based awards other than options, additional disclosures [Abstract]
Weighted-average remaining vesting period of share-based awards issued (in months)	2 months
Questar Gas [Member] | Minimum [Member]
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2014	$19.39
Questar Gas [Member] | Maximum [Member]
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2014	$19.39
Questar Pipeline [Member]
Transactions under the terms of the LTSIP - shares [Roll Forward]
Balance at December 31, 2014	25,269
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2014	$19.42
Share-based awards other than options, additional disclosures [Abstract]
Weighted-average remaining vesting period of share-based awards issued (in months)	3 months
Questar Pipeline [Member] | Minimum [Member]
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2014	$19.39
Questar Pipeline [Member] | Maximum [Member]
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2014	$19.56

ShareBased_Compensation_Restri1

Share-Based Compensation Restricted stock units (Details) (USD $)	12 Months Ended
	Dec. 31, 2014
Restricted Stock Units [Member]
Transactions under the terms of the LTSIP - shares [Roll Forward]
Balance at December 31, 2013	316,268
Granted	351,066
Vested	-131,007
Forfeited	-10,225
Balance at December 31, 2014	526,102
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2013	$23.62
Granted	$23.59
Vested	$23.64
Forfeited	$23.60
Balance at December 31, 2014	$23.60
Share-based awards other than options, additional disclosures [Abstract]
Weighted-average remaining vesting period of share-based awards issued (in months)	12 months
Number of shares of common stock to be issued for each vested restricted stock unit (RSU)	1
Vesting period for share-based award (in years)	3 years
Deferred Restricted Stock Units (RSUs) [Member]
Transactions under the terms of the LTSIP - shares [Roll Forward]
Balance at December 31, 2014	21,860
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2014	$23.61
Share-based awards other than options, additional disclosures [Abstract]
Weighted-average remaining vesting period of share-based awards issued (in months)	2 months
Number of shares of common stock to be issued for each vested restricted stock unit (RSU)	1
Vesting period for share-based award (in years)	1 year
Minimum [Member] | Restricted Stock Units [Member]
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2013	$22.17
Granted	$22.14
Vested	$22.17
Forfeited	$23.60
Balance at December 31, 2014	$22.14
Maximum [Member] | Restricted Stock Units [Member]
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2013	$25.44
Granted	$24.70
Vested	$25.44
Forfeited	$23.62
Balance at December 31, 2014	$24.70
Questar Gas [Member] | Restricted Stock Units [Member]
Transactions under the terms of the LTSIP - shares [Roll Forward]
Balance at December 31, 2014	82,351
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2014	$23.61
Share-based awards other than options, additional disclosures [Abstract]
Weighted-average remaining vesting period of share-based awards issued (in months)	12 months
Questar Gas [Member] | Minimum [Member] | Restricted Stock Units [Member]
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2014	$23.09
Questar Gas [Member] | Maximum [Member] | Restricted Stock Units [Member]
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2014	$23.62
Questar Pipeline [Member] | Restricted Stock Units [Member]
Transactions under the terms of the LTSIP - shares [Roll Forward]
Balance at December 31, 2014	95,841
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2014	$23.59
Share-based awards other than options, additional disclosures [Abstract]
Weighted-average remaining vesting period of share-based awards issued (in months)	12 months
Questar Pipeline [Member] | Minimum [Member] | Restricted Stock Units [Member]
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2014	$23.09
Questar Pipeline [Member] | Maximum [Member] | Restricted Stock Units [Member]
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2014	$23.62

ShareBased_Compensation_Perfor

Share-Based Compensation Performance shares (Details) (USD $)	12 Months Ended				3 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2015
Performance Shares [Member]
Major assumptions used in the calculation of fair value of performance shares granted [Abstract]
Weighted-average grant date fair value, performance shares granted (in dollars per share)	$31.07		$39.62	$25.42
Risk-free interest rate (in hundredths)	0.67%		0.40%	0.35%
Expected price volatility (in hundredths)	18.60%		19.00%	22.00%
Expected dividend yield (in hundredths)	3.05%		2.88%	3.35%
Expected life (in years and months)	2 years 11 months		2 years 11 months	2 years 11 months
Transactions under the terms of the LTSIP - shares [Roll Forward]
Balance at December 31, 2013	387,879
Granted	139,611
Payout adjustment	-76,657	[1]
Distributed	-51,104	[1]
Forfeited	-8,480
Balance at December 31, 2014	391,249	[2],[3]	387,879
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2013	$27.61
Granted	$31.07		$39.62	$25.42
Payout adjustment	$18.23	[1]
Distributed	$18.23	[1]
Forfeited	$33.33
Balance at December 31, 2014	$31.78	[2],[3]	$27.61
Performance shares, additional disclosures [Abstract]
Performance share payout adjustment multiplier	0.4
Performance period that the target number of performance shares for each exectutive officer is subject to adjustment based on the company's performance (in years)	3 years
Equity- and liability-based performance share compensation expense	$3.80		$2	$1.80
Weighted-average remaining vesting period of share-based awards issued (in months)	18 months
Performance share cash distributed	$0.60
Performance Shares [Member] | Vested Undistributed [Member]
Performance shares, additional disclosures [Abstract]
Performance share payout adjustment multiplier					1
Number of undistributed performance shares at end of period (in shares)	134,778
The date the share-based award will be distributed					first quarter of 2015
Performance Shares [Member] | Minimum [Member]
Major assumptions used in the calculation of fair value of performance shares granted [Abstract]
Weighted-average grant date fair value, performance shares granted (in dollars per share)	$31.07
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2013	$18.23
Granted	$31.07
Payout adjustment	$18.23	[1]
Distributed	$18.23	[1]
Forfeited	$25.42
Balance at December 31, 2014	$25.42	[2],[3]
Performance shares, additional disclosures [Abstract]
Performance share payout adjustment multiplier	0
Performance Shares [Member] | Maximum [Member]
Major assumptions used in the calculation of fair value of performance shares granted [Abstract]
Weighted-average grant date fair value, performance shares granted (in dollars per share)	$31.07
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2013	$39.62
Granted	$31.07
Payout adjustment	$18.23	[1]
Distributed	$18.23	[1]
Forfeited	$39.62
Balance at December 31, 2014	$39.62	[2],[3]
Performance shares, additional disclosures [Abstract]
Performance share payout adjustment multiplier	3
Performance Shares - Cash Settled [Member]
Performance shares, additional disclosures [Abstract]
Number of undistributed performance shares at end of period (in shares)	128,270
Performance Shares - Equity Settled [Member]
Performance shares, additional disclosures [Abstract]
Number of undistributed performance shares at end of period (in shares)	262,979
Questar Gas [Member] | Performance Shares [Member]
Transactions under the terms of the LTSIP - shares [Roll Forward]
Balance at December 31, 2014	33,826
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2014	$31.73
Performance shares, additional disclosures [Abstract]
Weighted-average remaining vesting period of share-based awards issued (in months)	18 months
Questar Gas [Member] | Performance Shares [Member] | Minimum [Member]
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2014	$25.42
Questar Gas [Member] | Performance Shares [Member] | Maximum [Member]
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2014	$39.62
Questar Pipeline [Member] | Performance Shares [Member]
Transactions under the terms of the LTSIP - shares [Roll Forward]
Balance at December 31, 2014	59,956
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2014	$31.59
Performance shares, additional disclosures [Abstract]
Weighted-average remaining vesting period of share-based awards issued (in months)	18 months
Questar Pipeline [Member] | Performance Shares [Member] | Minimum [Member]
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2014	$25.42
Questar Pipeline [Member] | Performance Shares [Member] | Maximum [Member]
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]
Balance at December 31, 2014	$39.62
[1]	Actual shares and cash distributed were determined by multiplying the target shares by 0.40 to reflect Questar's final total shareholder return ranking relative to the specified peer companies for the performance period from January 1, 2011 through DecemberB 31, 2013. The total amount paid for the cash half of the distribution was $0.6 million.
[2]	The total undistributed balance consists of 262,979 of share-settled target shares and 128,270 of cash-settled target shares.
[3]	The total undistributed balance includes 134,778 vested target shares that are expected to be distributed in the first quarter of 2015 using a payout adjustment multiplier of 1.00. The multiplier reflects Questar's final total shareholder return ranking relative to the specified peer companies for the performance period from January 1, 2012 through DecemberB 31, 2014. The distribution will be half in shares, half in cash. These target shares are included in the amounts described in note (2), immediately above.

Employee_Benefits_Defined_bene

Employee Benefits Defined benefit plans disclosures (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Total debt securities [Abstract]
Time over which set performance objectives for each investment manager are expected to be met (in years)	3 years
Minimum [Member]
Total debt securities [Abstract]
Average maturities for investment-grade intermediate-term debt (in years)	5 years
Average maturities for investment-grade long-term debt (in years)	10 years
Maximum [Member]
Total debt securities [Abstract]
Average maturities for investment-grade intermediate-term debt (in years)	10 years
Average maturities for below-investment-grade debt (in years)	10 years
Total domestic equity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total domestic equity securities (in hundredths)	38.00%	38.00%
Policy Range [Abstract]
Total domestic equity securities - minimum range (in hundredths)	35.00%	35.00%
Total domestic equity securities - maximum range (in hundredths)	45.00%	45.00%
Total foreign securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total domestic equity securities (in hundredths)	22.00%	25.00%
Policy Range [Abstract]
Total domestic equity securities - minimum range (in hundredths)	25.00%	25.00%
Total domestic equity securities - maximum range (in hundredths)	35.00%	35.00%
Developed market foreign equity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total domestic equity securities (in hundredths)	18.00%	20.00%
Emerging market foreign equity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total domestic equity securities (in hundredths)	4.00%	5.00%
Total debt securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total domestic equity securities (in hundredths)	30.00%	27.00%
Policy Range [Abstract]
Total domestic equity securities - minimum range (in hundredths)	25.00%	25.00%
Total domestic equity securities - maximum range (in hundredths)	35.00%	35.00%
Investment grade intermediate term debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Total domestic equity securities (in hundredths)	7.00%	7.00%
Investment grade long-term debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Total domestic equity securities (in hundredths)	14.00%	11.00%
Below-Investment grade debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Total domestic equity securities (in hundredths)	9.00%	9.00%
Inflation protection securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total domestic equity securities (in hundredths)	9.00%	9.00%
Policy Range [Abstract]
Total domestic equity securities - minimum range (in hundredths)	0.00%	0.00%
Total domestic equity securities - maximum range (in hundredths)	10.00%	10.00%
Cash and short-term investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Total domestic equity securities (in hundredths)	1.00%	1.00%
Policy Range [Abstract]
Total domestic equity securities - minimum range (in hundredths)	0.00%	0.00%
Total domestic equity securities - maximum range (in hundredths)	3.00%	3.00%

Employee_Benefits_Schedule_of_

Employee Benefits Schedule of fair value of financial assets for pension and postretirement benefits (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$737.20	$672
Corporate bonds [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	1.5	1.7
U.S. government securities [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	12.9	1.6
Fixed income registered investment companies [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	114.9	110.7
Inflation protection registered investment companies [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	34.5	29.4
Domestic equity registered investment companies [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	1.6	2
Cash equivalent commingled funds [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	6.7	3.5
Domestic equity index commingled funds [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	275.8	256.2
Foreign equity growth commingled funds [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	41.2	42.9
Foreign equity index commingled funds [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	77.8	76
Corporate debt commingled funds [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	91	72.1
Inflation protection commingled funds [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	33.1	28.8
Foreign equity growth 103-12 investment entities [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	46.2	47.1
Level 1 [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	151	142.1
Level 1 [Member] | Corporate bonds [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Level 1 [Member] | U.S. government securities [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Level 1 [Member] | Fixed income registered investment companies [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	114.9	110.7
Level 1 [Member] | Inflation protection registered investment companies [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	34.5	29.4
Level 1 [Member] | Domestic equity registered investment companies [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	1.6	2
Level 1 [Member] | Cash equivalent commingled funds [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Level 1 [Member] | Domestic equity index commingled funds [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Level 1 [Member] | Foreign equity growth commingled funds [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Level 1 [Member] | Foreign equity index commingled funds [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Level 1 [Member] | Corporate debt commingled funds [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Level 1 [Member] | Inflation protection commingled funds [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Level 1 [Member] | Foreign equity growth 103-12 investment entities [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Level 2 [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	586.2	529.9
Level 2 [Member] | Corporate bonds [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	1.5	1.7
Level 2 [Member] | U.S. government securities [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	12.9	1.6
Level 2 [Member] | Fixed income registered investment companies [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Level 2 [Member] | Inflation protection registered investment companies [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Level 2 [Member] | Domestic equity registered investment companies [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Level 2 [Member] | Cash equivalent commingled funds [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	6.7	3.5
Level 2 [Member] | Domestic equity index commingled funds [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	275.8	256.2
Level 2 [Member] | Foreign equity growth commingled funds [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	41.2	42.9
Level 2 [Member] | Foreign equity index commingled funds [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	77.8	76
Level 2 [Member] | Corporate debt commingled funds [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	91	72.1
Level 2 [Member] | Inflation protection commingled funds [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	33.1	28.8
Level 2 [Member] | Foreign equity growth 103-12 investment entities [Member]
Defined Benefit Plan Disclosure Fair Value [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$46.20	$47.10

Employee_Benefits_Pension_and_

Employee Benefits Pension and postretirement benefit plan obligation disclosures (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
	semimonthly_pay_periods
Change in plan assets [Roll Forward]
Fair value of plan assets at end of year	$737.20	$672
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of consecutive semimonthly pay periods of highest earnings, on which pension plan benefits are based	72
Number of final years of employment, during which the highest 72 consecutive semimonthly earnings periods for pension plan benefits are determined	10 years
Accumulated benefit obligation	708.9
Change in benefit obligation [Roll Forward]
Benefit obligation at beginning of year	702.7	760.3
Service cost	11.8	13.9	13.2
Interest cost	33.4	30.6	31.1
Change in plan assumptions	109.1	-73.6
Actuarial (gain)	-0.7	-7.7
Benefits paid	-22.3	-20.8
Benefit obligation at end of year	834	702.7	760.3
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	628.2	510.8
Actual gain on plan assets	40.4	80.2
Company contributions to the plan	43.9	58
Fair value of plan assets at end of year	690.2	628.2	510.8
Funded status of plan [Abstract]
Underfunded status (current and long-term)	-143.8	-74.5
Estimated benefit-plan payments for the next five years and subsequent five years aggregated [Abstract]
2015	67.3
2016	52.6
2017	47.2
2018	63
2019	53.6
2020 through 2024	265
Pension Plans, Defined Benefit, Qualified Only [Member]
Assumptions used to calculate benefit obligations [Abstract]
Discount rate used to calculate benefit obligation (in hundredths)	4.10%	4.90%
Rate of increase in compensation used to calculate benefit obligation (in hundredths)	5.50%	5.50%
Long-term return on assets used to calculate benefit obligation (in hundredths)	7.00%	7.25%
Funded status of plan [Abstract]
Projected pension funding in the next fiscal year	44
Pension Plans, Defined Benefit, Nonqualified Only [Member]
Assumptions used to calculate benefit obligations [Abstract]
Discount rate used to calculate benefit obligation (in hundredths)	2.90%	3.30%
Rate of increase in compensation used to calculate benefit obligation (in hundredths)	5.50%	5.50%
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Percent of 1992 contribution, to which the Company's portion of health-care benefits is generally limited for employees who retired after January 1, 1993	170.00%
Period of time over which the transition obligation was amortized that began in 1992 and ended in 2012 (in years)	20 years
Assumptions used to calculate benefit obligations [Abstract]
Discount rate used to calculate benefit obligation (in hundredths)	4.00%	4.70%
Long-term return on assets used to calculate benefit obligation (in hundredths)	7.00%	7.25%
Assumed health care cost trend rates used to calculate benefit obligation [Abstract]
Health-care inflation rate used to calculate benefit obligation (in hundredths)	7.50%	8.00%
Ultimate health care cost trend rate used to calculate benefit obligation (in hundredths)	4.50%	4.50%
Year that rate reaches ultimate trend rate, used to calculate benefit obligation	2021	2021
Change in benefit obligation [Roll Forward]
Benefit obligation at beginning of year	86.3	94.4
Service cost	0.6	0.7	0.7
Interest cost	3.8	3.7	3.8
Change in plan assumptions	4.9	-8.6
Actuarial (gain)	-3	-0.5
Benefits paid	-1.8	-3.4
Benefit obligation at end of year	90.8	86.3	94.4
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	43.8	37.6
Actual gain on plan assets	2.9	5.8
Company contributions to the plan	2.1	3.8
Fair value of plan assets at end of year	47	43.8	37.6
Funded status of plan [Abstract]
Underfunded status (current and long-term)	-43.8	-42.5
Estimated benefit-plan payments for the next five years and subsequent five years aggregated [Abstract]
2015	4.3
2016	4.5
2017	4.6
2018	4.8
2019	5
2020 through 2024	$26.30

Employee_Benefits_Net_pension_

Employee Benefits Net pension and postretirement benefit cost and other disclosures (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	$11.80	$13.90	$13.20
Interest cost	33.4	30.6	31.1
Expected return on plan assets	-43.6	-38.1	-31.8
Prior service and other costs	0.6	1.1	1.1
Recognized net actuarial loss	15.4	28.9	24.9
Accretion of regulatory liability	0	0	0
Net periodic cost	17.6	36.4	38.5
Assumed health care cost trend rates used to calculate net benefit costs [Abstract]
Estimated net benefit cost for next fiscal year	22.3
Estimated amortization of actuarial loss from AOCI for next fiscal year	22.7
Estimated amortization of prior service cost from AOCI for next fiscal year	0.1
Pension Plans, Defined Benefit, Qualified Only [Member]
Assumptions used to calculate net pension and postretirement benefit costs [Abstract]
Discount rate used to calculate net benefit costs (in hundredths)	4.90%	4.20%	4.80%
Rate of increase in compensation used to calculate net benefit costs (in hundredths)	5.50%	5.50%	5.50%
Long-term return on assets used to calculate net benefit costs (in hundredths)	7.25%	7.25%	7.25%
Pension Plans, Defined Benefit, Nonqualified Only [Member]
Assumptions used to calculate net pension and postretirement benefit costs [Abstract]
Discount rate used to calculate net benefit costs (in hundredths)	3.30%	2.40%	3.30%
Rate of increase in compensation used to calculate net benefit costs (in hundredths)	5.50%	5.50%	5.50%
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	0.6	0.7	0.7
Interest cost	3.8	3.7	3.8
Expected return on plan assets	-3	-2.6	-2.3
Prior service and other costs	0	0	1.6
Recognized net actuarial loss	0.7	3	2.5
Accretion of regulatory liability	0.8	0.5	0.3
Net periodic cost	2.9	5.3	6.6
Assumptions used to calculate net pension and postretirement benefit costs [Abstract]
Discount rate used to calculate net benefit costs (in hundredths)	4.70%	4.00%	4.40%
Long-term return on assets used to calculate net benefit costs (in hundredths)	7.25%	7.25%	7.25%
Assumed health care cost trend rates used to calculate net benefit costs [Abstract]
Health-care inflation rate used to calculate net benefit costs (in hundredths)	8.00%	8.50%	7.00%
Ultimate health care cost trend rate used to calculate net benefit costs (in hundredths)	4.50%	4.50%	5.00%
Year that rate reaches ultimate trend rate, used to calculate net benefit costs	2021	2021	2014
Estimated net benefit cost for next fiscal year	2.2
Estimated amortization of actuarial loss from AOCI for next fiscal year	1.3
Effect of 1% increase in the health care inflation rate on the yearly service and interest cost	0
Effect of 1% increase in the health care inflation rate on the accumulated post-retirement benefit obligation	0.8
Effect of 1% decrease in the health care inflation rate on the yearly service and interest cost	0
Effect of 1% decrease in the health care inflation rate on the accumulated post-retirement benefit obligation	($0.70)

Employee_Benefits_Subsidiary_p

Employee Benefits Subsidiary plan participation and 401(k) Retirement Income Plan (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Percentage of employees' qualifying earnings to which the employer matching contribution applies (in hundredths)	100.00%
Percentage of employees' qualifying earnings matched by the employer for pre-tax 401(k) Plan purchases (in hundredths)	6.00%
Percentage of employees' qualifying prior year earnings contributed by the employer on a non-matching basis for pre-tax 401(k) Plan purchases (in hundredths)	4.00%
Yearly 401(k) Plan contribution expense amount recognized in the current period	$9.10	$7.20	$7.30
Pension Plans, Defined Benefit [Member]
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Net periodic cost	17.6	36.4	38.5
Other Postretirement Benefit Plans, Defined Benefit [Member]
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Net periodic cost	2.9	5.3	6.6
Questar Gas [Member]
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Yearly 401(k) Plan contribution expense amount recognized in the current period	4.1	3.4	3.6
Questar Gas [Member] | Pension Plans, Defined Benefit [Member]
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Net periodic cost	8.5	18.1	19.7
Share of total defined benefit plan contributions	21.8	29.6	39.4
Questar Gas [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Net periodic cost	0.8	2.4	3.3
Share of total defined benefit plan contributions	1.1	2	1.9
Questar Pipeline [Member]
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Yearly 401(k) Plan contribution expense amount recognized in the current period	1.8	1.4	1.4
Questar Pipeline [Member] | Pension Plans, Defined Benefit [Member]
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Net periodic cost	3.2	6.7	7.4
Share of total defined benefit plan contributions	8.1	11.2	15.1
Questar Pipeline [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Net periodic cost	-0.4	0.2	1.2
Share of total defined benefit plan contributions	$0.40	$0.70	$0.60

Operations_by_Line_of_Business2

Operations by Line of Business (Details) (USD $)	3 Months Ended								12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
									lines_of_business
Segment Reporting Information [Line Items]
Number of complementary lines of business									3
Revenues [Abstract]
Total Revenues	$373.20	$157.90	$201.30	$456.90	$397.90	$149.60	$195.60	$476.90	$1,189.30	$1,220	$1,098.90
Operating Expenses [Abstract]
Cost of sales									186.3	285.9	192.3
Operating and maintenance									189	174.3	180.8
General and administrative									127.9	121	120.8
Retirement incentive									0	0	4.9
Production and other taxes									66.2	57.4	47.9
Depreciation, depletion and amortization									213.7	194.8	181.6
Abandonment and impairment					0	80.6	0	0	2	80.6	0
Other operating expenses										0	0
Total Operating Expenses									785.1	914	728.3
Net gain (loss) from asset sales									1.2	-0.2	5.1
Operating Income (Loss)	111.7	68.9	76.5	148.3	122.6	-19.4	71.2	131.4	405.4	305.8	375.7
Interest and other income									6.6	9.9	7
Income from unconsolidated affiliate									3.5	3.7	3.7
Interest expense									-63.1	-56.9	-57.9
Income taxes									-125.9	-101.3	-116.5
Net Income (Loss)	62.5	38.6	40.3	85.1	68.1	-19.2	39.4	72.9	226.5	161.2	212
Additional Line-of-Business Information [Abstract]
Identifiable assets	4,249.70				4,054.30				4,249.70	4,054.30	3,803.30
Goodwill	9.8				9.8				9.8	9.8	9.8
Investment in unconsolidated affiliate	24.7				25.6				24.7	25.6	26.5
Cash capital expenditures, including acquisitions									371.5	503.7	370.7
Accrued capital expenditures, including acquisitions									359.7	505.6	366.3
From unaffiliated customers [Member]
Revenues [Abstract]
Total Revenues									1,189.30	1,220	1,098.90
From affiliated companies [Member]
Revenues [Abstract]
Total Revenues									0	0	0
Operating Segments [Member] | Questar Gas [Member]
Revenues [Abstract]
Total Revenues									960.9	985.8	862.2
Operating Expenses [Abstract]
Cost of sales									604.8	650.6	533.3
Operating and maintenance									122.5	113.1	119
General and administrative									52.8	52.5	51.2
Retirement incentive											2.4
Production and other taxes									17.8	18	16.2
Depreciation, depletion and amortization									53.6	49.7	47.2
Abandonment and impairment									0	0
Other operating expenses										0	0
Total Operating Expenses									851.5	883.9	769.3
Net gain (loss) from asset sales									0.1	0	0
Operating Income (Loss)									109.5	101.9	92.9
Interest and other income									5.9	5.1	5.5
Income from unconsolidated affiliate									0	0	0
Interest expense									-28.2	-22.3	-24.1
Income taxes									-32	-31.9	-27.2
Net Income (Loss)									55.2	52.8	47.1
Additional Line-of-Business Information [Abstract]
Identifiable assets	1,911.10				1,762				1,911.10	1,762	1,593.50
Goodwill	5.6				5.6				5.6	5.6	5.6
Investment in unconsolidated affiliate	0				0				0	0	0
Cash capital expenditures, including acquisitions									174.7	166.2	162.1
Accrued capital expenditures, including acquisitions									161.5	177.3	161.2
Operating Segments [Member] | Questar Gas [Member] | From unaffiliated customers [Member]
Revenues [Abstract]
Total Revenues									960.9	985.2	859.7
Operating Segments [Member] | Questar Gas [Member] | From affiliated companies [Member]
Revenues [Abstract]
Total Revenues									0	0.6	2.5
Operating Segments [Member] | Wexpro [Member]
Revenues [Abstract]
Total Revenues									385.9	339.9	310.2
Operating Expenses [Abstract]
Cost of sales									0	0	0
Operating and maintenance									31.5	28.6	26.8
General and administrative									32.6	28.7	26.8
Retirement incentive											0.2
Production and other taxes									37.6	28.3	20.8
Depreciation, depletion and amortization									100.5	85.8	77.4
Abandonment and impairment									2	0
Other operating expenses										0.6	2.5
Total Operating Expenses									204.2	172	154.5
Net gain (loss) from asset sales									1.6	-0.2	2.4
Operating Income (Loss)									183.3	167.7	158.1
Interest and other income									1.1	5	2.8
Income from unconsolidated affiliate									0	0	0
Interest expense									-0.1	-0.1	0
Income taxes									-61.5	-62	-57
Net Income (Loss)									122.8	110.6	103.9
Additional Line-of-Business Information [Abstract]
Identifiable assets	977.5				993.8				977.5	993.8	836.1
Goodwill	0				0				0	0	0
Investment in unconsolidated affiliate	0				0				0	0	0
Cash capital expenditures, including acquisitions									114.4	249.5	144.5
Accrued capital expenditures, including acquisitions									114.7	240.7	145.5
Operating Segments [Member] | Wexpro [Member] | From unaffiliated customers [Member]
Revenues [Abstract]
Total Revenues									35.6	45.1	36.1
Operating Segments [Member] | Wexpro [Member] | From affiliated companies [Member]
Revenues [Abstract]
Total Revenues									350.3	294.8	274.1
Operating Segments [Member] | Questar Pipeline [Member]
Revenues [Abstract]
Total Revenues									263.9	266.2	277.5
Operating Expenses [Abstract]
Cost of sales									4	6.1	6.7
Operating and maintenance									34.1	32.5	35.2
General and administrative									44.1	46.7	50
Retirement incentive											0.9
Production and other taxes									9.1	9.3	9.1
Depreciation, depletion and amortization									54.5	55.5	54.3
Abandonment and impairment									0	80.6
Other operating expenses										0	0
Total Operating Expenses									145.8	230.7	156.2
Net gain (loss) from asset sales									-0.5	0	2.7
Operating Income (Loss)									117.6	35.5	124
Interest and other income									1.2	1.8	0.6
Income from unconsolidated affiliate									3.5	3.7	3.7
Interest expense									-26.1	-25.8	-26.3
Income taxes									-35.6	-7	-37.3
Net Income (Loss)									60.6	8.2	64.7
Additional Line-of-Business Information [Abstract]
Identifiable assets	1,232.40				1,222.80				1,232.40	1,222.80	1,298.90
Goodwill	4.2				4.2				4.2	4.2	4.2
Investment in unconsolidated affiliate	24.7				25.6				24.7	25.6	26.5
Cash capital expenditures, including acquisitions									59.4	73.4	60.6
Accrued capital expenditures, including acquisitions									61.3	70	58.6
Operating Segments [Member] | Questar Pipeline [Member] | From unaffiliated customers [Member]
Revenues [Abstract]
Total Revenues									190.2	189.5	203.1
Operating Segments [Member] | Questar Pipeline [Member] | From affiliated companies [Member]
Revenues [Abstract]
Total Revenues									73.7	76.7	74.4
Corporate and Other [Member]
Revenues [Abstract]
Total Revenues									2.6	0.2	0
Operating Expenses [Abstract]
Cost of sales									0.9	0.1	0
Operating and maintenance									0.9	0.1	0
General and administrative									-1	-6.3	-6.6
Retirement incentive											1.4
Production and other taxes									1.7	1.8	1.8
Depreciation, depletion and amortization									5.1	3.8	2.7
Abandonment and impairment									0	0
Other operating expenses										0	0
Total Operating Expenses									7.6	-0.5	-0.7
Net gain (loss) from asset sales									0	0	0
Operating Income (Loss)									-5	0.7	0.7
Interest and other income									-1.2	-1.3	-1.2
Income from unconsolidated affiliate									0	0	0
Interest expense									-9.1	-9.4	-8.2
Income taxes									3.2	-0.4	5
Net Income (Loss)									-12.1	-10.4	-3.7
Additional Line-of-Business Information [Abstract]
Identifiable assets	128.7				75.7				128.7	75.7	74.8
Goodwill	0				0				0	0	0
Investment in unconsolidated affiliate	0				0				0	0	0
Cash capital expenditures, including acquisitions									23	14.6	3.5
Accrued capital expenditures, including acquisitions									22.2	17.6	1
Corporate and Other [Member] | From unaffiliated customers [Member]
Revenues [Abstract]
Total Revenues									2.6	0.2	0
Corporate and Other [Member] | From affiliated companies [Member]
Revenues [Abstract]
Total Revenues									0	0	0
Interco. Trans. [Member]
Revenues [Abstract]
Total Revenues									-424	-372.1	-351
Operating Expenses [Abstract]
Cost of sales									-423.4	-370.9	-347.7
Operating and maintenance									0	0	-0.2
General and administrative									-0.6	-0.6	-0.6
Retirement incentive											0
Production and other taxes									0	0	0
Depreciation, depletion and amortization									0	0	0
Abandonment and impairment									0	0
Other operating expenses										-0.6	-2.5
Total Operating Expenses									-424	-372.1	-351
Net gain (loss) from asset sales									0	0	0
Operating Income (Loss)									0	0	0
Interest and other income									-0.4	-0.7	-0.7
Income from unconsolidated affiliate									0	0	0
Interest expense									0.4	0.7	0.7
Income taxes									0	0	0
Net Income (Loss)									0	0	0
Additional Line-of-Business Information [Abstract]
Identifiable assets	0				0				0	0	0
Goodwill	0				0				0	0	0
Investment in unconsolidated affiliate	0				0				0	0	0
Cash capital expenditures, including acquisitions									0	0	0
Accrued capital expenditures, including acquisitions									0	0	0
Interco. Trans. [Member] | From unaffiliated customers [Member]
Revenues [Abstract]
Total Revenues									0	0	0
Interco. Trans. [Member] | From affiliated companies [Member]
Revenues [Abstract]
Total Revenues									($424)	($372.10)	($351)

RelatedParty_Transaction_Detai

Related-Party Transaction (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Related Party Transaction [Line Items]
Oil and NGL income sharing (in millions)	$0	$0.60	$2.50
Questar Gas [Member] | Sales [Member] | Questar Pipeline [Member]
Related Party Transaction [Line Items]
Related Party Revenue	73.3	76.3	74
Questar Gas [Member] | General and Administrative Expense [Member] | Questar Pipeline [Member]
Related Party Transaction [Line Items]
Related Party Expense	3.8	3.5	8.8
Questar Corp [Member] | General and Administrative Expense [Member] | Questar Gas [Member]
Related Party Transaction [Line Items]
Related Party Expense	47.8	48.4	49.3
Questar Corp [Member] | General and Administrative Expense [Member] | Questar Pipeline [Member]
Related Party Transaction [Line Items]
Related Party Expense	25.8	28.2	24.5
Questar Corp [Member] | Interest Expense [Member] | Questar Gas [Member]
Related Party Transaction [Line Items]
Related Party Expense	0.1	0.5	0.6
Questar Corp [Member] | Interest Income [Member] | Questar Pipeline [Member]
Related Party Transaction [Line Items]
Related Party Revenue	0.1	0.2	0.1
Affiliated Entity [Member] | Rent Expense [Member] | Questar Gas [Member]
Related Party Transaction [Line Items]
Related Party Expense			0.4
Affiliated Entity [Member] | Rent Expense [Member] | Questar Pipeline [Member]
Related Party Transaction [Line Items]
Related Party Expense			0.3
Questar And Other Affiliates [Member] | General and Administrative Expense [Member] | Questar Gas [Member]
Related Party Transaction [Line Items]
Related Party Expense	6.1	6.7	13.6
Questar And Other Affiliates [Member] | General and Administrative Expense [Member] | Questar Pipeline [Member]
Related Party Transaction [Line Items]
Related Party Expense	5	4.9
Questar Pipeline [Member] | Questar Gas [Member]
Related Party Transaction [Line Items]
Amount of Mdth per day reserved transportation capacity on Questar Pipeline - peak months	916
Amount of Mdth per day reserved transportation capacity on Questar Pipeline - off-peak months	841
Questar Pipeline [Member] | General and Administrative Expense [Member] | Questar Gas [Member]
Related Party Transaction [Line Items]
Related Party Expense	3.7	3.7
Questar Pipeline [Member] | Cost of Sales [Member] | Questar Gas [Member]
Related Party Transaction [Line Items]
Related Party Expense	72.9	73	73.6
Wexpro [Member] | Sales [Member] | Questar Gas [Member]
Related Party Transaction [Line Items]
Oil and NGL income sharing (in millions)	0	0.6	2.5
Wexpro [Member] | Cost of Sales [Member] | Questar Gas [Member]
Related Party Transaction [Line Items]
Related Party Expense	$349.70	$294.60	$274

Asset_Impairments_Details

Asset Impairments (Details) (USD $)	3 Months Ended				12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property, Plant and Equipment [Line Items]
Asset impairment charge, before income taxes	$0	$80.60	$0	$0	$2	$80.60	$0
Wexpro [Member]
Property, Plant and Equipment [Line Items]
Asset impairment charge, before income taxes					2
Wexpro [Member] | Wexpro Brady Field [Member]
Property, Plant and Equipment [Line Items]
Asset impairment charge, before income taxes					2
Questar Pipeline [Member]
Property, Plant and Equipment [Line Items]
Asset impairment charge, before income taxes					0	80.6	0
Questar Pipeline [Member] | Questar Southern Trails Pipeline - Eastern Segment [Member]
Property, Plant and Equipment [Line Items]
Asset impairment charge, before income taxes						80.6
After-tax asset impairment charge						$52.40

Wexpro_Acquisitions_of_Produci2

Wexpro Acquisitions of Producing Properties (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Canyon Creek [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition	17-Dec-14
Business Acquisition, Description of Acquired Entity	an additional interest in natural gas-producing properties in existing Wexpro-operated assets in the Canyon Creek Unit of southwestern Wyoming's Vermillion Basin
Business Combination, Consideration Transferred [Abstract]
Net cash paid	$52.40
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]
Proved properties	54.1
Asset retirement obligations	-1.7
Total fair value of identifiable net assets acquired	52.4
Business Combination, Separately recognized Transactions, Additional Disclosures [Abstract]
Acquisition Cost Expensed	0.3
Acquisition Cost Expensed, Financial Statement Caption	general and administrative expense
Trail [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition		4-Sep-13
Business Acquisition, Date of Acquisition Agreement		25-Jul-13
Business Combination, Consideration Transferred [Abstract]
Net cash paid		$104.30

Quarterly_Financial_Informatio2

Quarterly Financial Information - Unaudited (Details) (USD $)	3 Months Ended								12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Selected Quarterly Financial Information [Abstract]
Revenues	$373.20	$157.90	$201.30	$456.90	$397.90	$149.60	$195.60	$476.90	$1,189.30	$1,220	$1,098.90
Operating income (loss)	111.7	68.9	76.5	148.3	122.6	-19.4	71.2	131.4	405.4	305.8	375.7
Net income (loss)	62.5	38.6	40.3	85.1	68.1	-19.2	39.4	72.9	226.5	161.2	212
Asset impairment charge, before income taxes					$0	$80.60	$0	$0	$2	$80.60	$0
Basic earnings (loss) per common share	$0.36	$0.22	$0.23	$0.48	$0.39	($0.11)	$0.22	$0.42	$1.29	$0.92	$1.20
Diluted earnings (loss) per common share	$0.35	$0.22	$0.23	$0.48	$0.39	($0.11)	$0.22	$0.41	$1.29	$0.92	$1.19

Supplemental_Gas_and_Oil_Infor2

Supplemental Gas and Oil Information - Unaudited Capitalized Cost (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Capitalized Costs Relating to Gas and Oil Producing Activities, by Segment [Line Items]
Total Wexpro capitalized costs	$1,688.40	$1,626.40
Accumulated depreciation, depletion and amortization	-757.3	-689.8
Net capitalized costs	937.5	943.8
Wexpro [Member]
Capitalized Costs Relating to Gas and Oil Producing Activities, by Segment [Line Items]
Proved properties	1,675.60	1,625.50
Unproved properties	12.8	0.9
Total Wexpro capitalized costs	1,688.40	1,626.40
Accumulated depreciation, depletion and amortization	-757.3	-689.8
Net capitalized costs	931.1	936.6
Questar Gas [Member]
Capitalized Costs Relating to Gas and Oil Producing Activities, by Segment [Line Items]
Net capitalized costs	$6.40	$7.20

Supplemental_Gas_and_Oil_Infor3

Supplemental Gas and Oil Information - Unaudited Costs Incurred (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property Acquisition [Abstract]
Unproved	$11.90	$0.30	$0.60
Proved	54.1	106.4	0
Exploration (capitalized and expensed)	1.6	0	0
Development	49.1	133.1	146.9
Total costs incurred	116.7	239.8	147.5
Cost incurred to develop proved undeveloped reserves	$28.90	$106.30	$50.40

Supplemental_Gas_and_Oil_Infor4

Supplemental Gas and Oil Information - Unaudited Results of Operations for Oil and Gas Producing Activities (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Revenues [Abstract]
From unaffiliated customers	$35.60		$45.10		$36.10
From affiliated company	350.3	[1]	294.8	[1]	274.1	[1]
Total revenues	385.9		339.9		310.2
Production costs	67.5		57.5		50.1
Exploration expenses	1.6		0		0
Depreciation, depletion and amortization	100.5		85.8		77.4
Abandonment and impairment	2		0		0
Total expenses	171.6		143.3		127.5
Revenues less expenses	214.3		196.6		182.7
Income taxes	-71.4		-70.5		-64.8
Results of operations for gas- and oil-producing activities (excluding corporate overhead and interest costs)	142.9		126.1		117.9
Reimbursement of general and administrative expenses from Questar Gas included in affiliated-company revenues	$30.50		$27.50		$26.50
[1]	Primarily represents revenues received from Questar Gas pursuant to the Wexpro agreements. Revenues include reimbursement of general and administrative expenses amounting to $30.5 million in 2014, $27.5 million in 2013 and $26.5 million in 2012.

Supplemental_Gas_and_Oil_Infor5

Supplemental Gas and Oil Information - Unaudited Proved Developed and Undeveloped Oil and Gas Reserve Quantities (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
	Mcf	Mcf	Mcf	Mcf
Reserve Quantities [Line Items]
Number of years for development plan	5 years
Percent of prior year-end proved undeveloped reserves converted to developed status (in hundredths)	7.00%	42.00%	15.00%
Natural Gas (in Mcf) [Member]
Proved Reserves [Abstract]
Beginning balance (Mcf/bbl)	811,200,000	697,200,000	758,700,000
Revisions of previous estimates (Mcf/bbl)	-220,200,000	-112,800,000	-158,400,000
Extensions and discoveries (Mcf/bbl)	4,000,000	153,500,000	154,400,000
Purchase of reserves in place (Mcf/bbl)	35,900,000	133,900,000
Sale of reserves in place (Mcf/bbl)	-500,000
Production (Mcf/bbl)	-64,300,000	-60,600,000	-57,500,000
Ending balance (Mcf/bbl)	566,100,000	811,200,000	697,200,000
Proved Developed Reserves [Abstract]
Proved Developed Reserves (Mcf/bbl)	552,900,000	560,000,000	523,900,000	491,200,000
Proved Undeveloped Reserves [Abstract]
Proved Undeveloped Reserves (Mcf/bbl)	13,200,000	251,200,000	173,300,000	267,500,000
Oil and NGL (in bbl) [Member]
Proved Reserves [Abstract]
Beginning balance (Mcf/bbl)	5,617,000	6,169,000	5,344,000
Revisions of previous estimates (Mcf/bbl)	-442,000	-1,348,000	-122,000
Extensions and discoveries (Mcf/bbl)	205,000	857,000	1,612,000
Purchase of reserves in place (Mcf/bbl)	157,000	556,000
Sale of reserves in place (Mcf/bbl)	-219,000
Production (Mcf/bbl)	-587,000	-617,000	-665,000
Ending balance (Mcf/bbl)	4,731,000	5,617,000	6,169,000
Proved Developed Reserves [Abstract]
Proved Developed Reserves (Mcf/bbl)	4,678,000	4,384,000	4,967,000	4,420,000
Proved Undeveloped Reserves [Abstract]
Proved Undeveloped Reserves (Mcf/bbl)	53,000	1,233,000	1,202,000	924,000
Natural Gas Equivalents (in Mcfe) [Member]
Proved Reserves [Abstract]
Beginning balance (Mcfe)	844,900,000	734,200,000	790,700,000
Revisions of previous estimates (Mcfe)	-222,700,000	-120,800,000	-159,000,000
Extensions and discoveries (Mcfe)	5,200,000	158,600,000	164,000,000
Purchase of reserves in place (Mcfe)	36,800,000	137,200,000
Sale of reserves in place (Mcfe)	-1,900,000
Production (Mcfe)	-67,800,000	-64,300,000	-61,500,000
Ending balance (Mcfe)	594,500,000	844,900,000	734,200,000
Proved Developed Reserves [Abstract]
Proved Developed Reserves (Mcfe)	581,000,000	586,300,000	553,700,000	517,700,000
Proved Undeveloped Reserves [Abstract]
Proved Undeveloped Reserve (Mcfe)	13,500,000	258,600,000	180,500,000	273,000,000

Supplemental_Gas_and_Oil_Infor6

Supplemental Gas and Oil Information - Unaudited Standardized Measure of Discounted Future Net Cash Flows (Details) (Non-Cost-of-Service Gas and Oil Reserves [Member], USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014
	Mcfe
Non-Cost-of-Service Gas and Oil Reserves [Member]
Discounted Future Net Cash Flows Relating to Non-Cost-of-Service Proved Reserves [Line Items]
Ending non-cost-of-service proved reserves (Mcfe)	36,600,000
Standardized Measure of Discounted Future Net Cash Flows Relating to Non-Cost-of-Service Proved Reserves [Abstract]
Future cash inflows	$190.70
Future production costs	-65.9
Future income tax expenses	-43.7
Future net cash flows	81.1
10% annual discount for estimated timing of net cash flows	-34.4
Standardized measure of discounted future net cash flows	46.7
Principal Sources of Change in Standardized Measure of Discounted Future Net Cash Flow Relating to Non-Cost-of-Service Proved Reserves [Abstract]
Balance at beginning of year	0
Net increase due to purchases of reserves in place	46.8
Sales of gas and oil produced during the period, net of production costs	-1.5
Net change in prices and production costs related to future production	0.8
Accretion of discount	0.6
Net Change	46.7
Balance at end of year	$46.70

Schedule_of_Valuation_and_Qual1

Schedule of Valuation and Qualifying Accounts (Details) (Allowance for Bad Debt [Member], USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning Balance	$1.70	$3.10	$3.20
Amounts charged to expense	1.7	0.2	1.2
Deductions for accounts written off and other	-1.7	-1.6	-1.3
Ending Balance	1.7	1.7	3.1
Questar Gas [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning Balance	1.4	2.8	2.9
Amounts charged to expense	1.7	0.2	1.1
Deductions for accounts written off and other	-1.7	-1.6	-1.2
Ending Balance	1.4	1.4	2.8
Questar Pipeline [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning Balance	0.3	0.3	0.3
Amounts charged to expense	0	0	0.1
Deductions for accounts written off and other	0	0	-0.1
Ending Balance	$0.30	$0.30	$0.30