Debt Long-term Debt (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Total long-term debt outstanding	$ 1,244.6us-gaap_DebtInstrumentCarryingAmount	$ 1,244.6us-gaap_DebtInstrumentCarryingAmount
Less current portion	(25.1)us-gaap_DebtCurrent	0us-gaap_DebtCurrent
Less unamortized debt discount	(1.9)	(2.1)
Plus unamortized debt premium	0.5	0.7
Plus fair value hedge adjustment	2.0	3.9
Total long-term debt, less current portion	1,220.1us-gaap_LongTermDebtNoncurrent	1,247.1us-gaap_LongTermDebtNoncurrent
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	Feb. 01, 2016
Total long-term debt outstanding	250.0	250.0
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	Oct. 10, 2017
Debt Instrument, Maturity Date Range, End	Mar. 15, 2018
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	Apr. 01, 2018
Total long-term debt outstanding	50.0	50.0
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	Dec. 01, 2024
Total long-term debt outstanding	40.0	40.0
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	Dec. 01, 2027
Total long-term debt outstanding	110.0	110.0
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	Apr. 01, 2038
Total long-term debt outstanding	100.0	100.0
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	Dec. 01, 2043
Debt instrument, face amount		90.0
Debt Instrument, Term		30 years
Total long-term debt outstanding	90.0	90.0
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	Dec. 01, 2048
Debt instrument, face amount		60.0
Debt Instrument, Term		35 years
Total long-term debt outstanding	60.0	60.0
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		149.0
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	Oct. 15, 2015
Debt Instrument, Maturity Date Range, End	Dec. 11, 2018
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	Feb. 01, 2018
Total long-term debt outstanding	250.0	250.0
Questar Pipeline [Member] \| 4.875% Notes due 2041 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.875%
Debt Instrument, Maturity Date	Dec. 01, 2041
Total long-term debt outstanding	$ 180.0	$ 180.0

Net Proceeds from Issuance of Long-Term Debt

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Amount of short-term debt and current maturity of long-term debt and capital lease obligations due within one year or the normal operating cycle, if longer.

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Amount of long-term debt before deduction of unamortized discount or premium. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, with initial maturities beyond one year or the normal operating cycle, if longer.

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Face (par) amount of debt instrument at time of issuance.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Maximum contractual interest rate for funds borrowed, under the debt agreement.

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Minimum contractual interest rate for funds borrowed, under the debt agreement.

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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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Latest date the outstanding debt instruments are required to be repaid, in CCYY-MM-DD format.

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Earliest date the outstanding debt instruments are required to be repaid, in CCYY-MM-DD format.

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Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The amount of debt discount that was originally recognized at the issuance of the instrument that has yet to be amortized.

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The amount of debt premium that was originally recognized at the issuance of the instrument that has yet to be amortized.

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Amount as of the balance sheet date of the unamortized adjustment to the carrying value of an interest-bearing hedged item made under an effective fair value hedge that is amortized upon discontinuation of the fair value hedge.

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Carrying amount of long-term debt, net of unamortized discount or premium, excluding amounts to be repaid within one year or the normal operating cycle, if longer (current maturities). Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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