Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2015

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 Quarterly Report:

	Hierarchy Level of Fair Value Estimates	Carrying Amount		Estimated Fair Value		Carrying Amount		Estimated Fair Value		Carrying Amount		Estimated Fair Value
		Sept. 30, 2015				Sept. 30, 2014				Dec. 31, 2014
		(in millions)
Financial assets
Cash and cash equivalents	1	$	—	$	—	$	7.3	$	7.3	$	32.0	$	32.0
Long-term investment	1	14.0		14.0		15.8		15.8		15.7		15.7
Financial liabilities
Checks outstanding in excess of cash balances	1	2.8		2.8		—		—		—		—
Short-term debt	1	305.0		305.0		234.0		234.0		347.0		347.0
Long-term debt, including current portion	2	1,243.8		1,323.5		1,245.7		1,364.0		1,245.2		1,356.1

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 Quarterly Report:

	Hierarchy Level of Fair Value Estimates	Carrying Amount		Estimated Fair Value		Carrying Amount		Estimated Fair Value		Carrying Amount		Estimated Fair Value
		Sept. 30, 2015				Sept. 30, 2014				Dec. 31, 2014
		(in millions)
Financial assets
Cash and cash equivalents	1	$	—	$	—	$	—	$	—	$	19.8	$	19.8
Financial liabilities
Checks outstanding in excess of cash balances	1	3.4		3.4		2.1		2.1		—		—
Notes payable to Questar	1	180.0		180.0		90.8		90.8		119.3		119.3
Long-term debt	2	534.5		588.7		534.5		602.8		534.5		607.2

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 Quarterly Report:

	Hierarchy Level of Fair Value Estimates	Carrying Amount		Estimated Fair Value		Carrying Amount		Estimated Fair Value		Carrying Amount		Estimated Fair Value
		Sept. 30, 2015				Sept. 30, 2014				Dec. 31, 2014
		(in millions)
Financial assets
Cash and cash equivalents	1	$	1.1	$	1.1	$	0.4	$	0.4	$	7.4	$	7.4
Notes receivable from Questar	1	48.2		48.2		35.2		35.2		40.1		40.1
Financial liabilities
Long-term debt, including current portion	2	458.7		484.1		458.8		505.0		458.8		495.5