Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011
Carrying Amount [Member]
Financial assets [Abstract]
Cash and cash equivalents	$ 0	$ 11.6	$ 0
Long-term investment	15.0	14.4	14.1
Financial liabilities [Abstract]
Checks outstanding in excess of cash balances	5.4	0	9.6
Short-term debt	263.0	219.0	278.0
Long-term debt, including current portion	1,057.9	1,084.5	906.4
Estimated Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	0	11.6	0
Long-term investment	15.0	14.4	14.1
Financial liabilities [Abstract]
Checks outstanding in excess of cash balances	5.4	0	9.6
Short-term debt	263.0	219.0	278.0
Estimated Fair Value [Member] \| Fair Value, Inputs, Level 2 [Member]
Financial liabilities [Abstract]
Long-term debt, including current portion	1,237.5	1,223.4	1,013.9
Questar Gas [Member] \| Carrying Amount [Member]
Financial assets [Abstract]
Cash and cash equivalents	0	5.0	0
Financial liabilities [Abstract]
Checks outstanding in excess of cash balances	2.0	0	3.2
Notes payable to Questar	206.6	142.7	97.9
Long-term debt, including current portion	343.0	368.0	368.0
Questar Gas [Member] \| Estimated Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	0	5.0	0
Financial liabilities [Abstract]
Checks outstanding in excess of cash balances	2.0	0	3.2
Notes payable to Questar	206.6	142.7	97.9
Questar Gas [Member] \| Estimated Fair Value [Member] \| Fair Value, Inputs, Level 2 [Member]
Financial liabilities [Abstract]
Long-term debt, including current portion	426.3	445.4	431.7
Questar Pipeline [Member] \| Carrying Amount [Member]
Financial assets [Abstract]
Cash and cash equivalents	0.4	3.4	0.4
Notes receivable from Questar	42.0	8.8	0
Financial liabilities [Abstract]
Notes payable to Questar	0	0	130.3
Long-term debt, including current portion	458.9	459.0	280.9
Questar Pipeline [Member] \| Estimated Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	0.4	3.4	0.4
Notes receivable from Questar	42.0	8.8	0
Financial liabilities [Abstract]
Notes payable to Questar	0	0	130.3
Questar Pipeline [Member] \| Estimated Fair Value [Member] \| Fair Value, Inputs, Level 2 [Member]
Financial liabilities [Abstract]
Long-term debt, including current portion	$ 547.8	$ 519.4	$ 329.1

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the Company may deposit additional funds at any time and also effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months.

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