Fair Value Measurements (Details) - USD ($)	3 Months Ended					12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Sep. 30, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Jun. 30, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Abandonment and impairment						$ 12,500,000	$ 2,000,000	$ 80,600,000
Carrying Amount [Member]
Financial assets [Abstract]
Cash and cash equivalents	$ 25,000,000					25,000,000	32,000,000
Long-term investment	14,100,000					14,100,000	15,700,000
Financial liabilities [Abstract]
Short-term debt	457,600,000					457,600,000	347,000,000
Long-term debt, including current portion	1,218,300,000					1,218,300,000	1,245,200,000
Estimated Fair Value [Member] | Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	25,000,000					25,000,000	32,000,000
Financial liabilities [Abstract]
Short-term debt	457,600,000					457,600,000	347,000,000
Estimated Fair Value [Member] | Level 2 [Member]
Financial liabilities [Abstract]
Long-term debt, including current portion	1,263,700,000					1,263,700,000	1,356,100,000
Estimated Fair Value [Member] | Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Financial assets [Abstract]
Long-term investment	14,100,000					14,100,000	15,700,000
Questar Gas [Member] | Carrying Amount [Member]
Financial assets [Abstract]
Cash and cash equivalents	10,500,000					10,500,000	19,800,000
Financial liabilities [Abstract]
Notes payable to Questar	273,300,000					273,300,000	119,300,000
Long-term debt, including current portion	534,500,000					534,500,000	534,500,000
Questar Gas [Member] | Estimated Fair Value [Member] | Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	10,500,000					10,500,000	19,800,000
Financial liabilities [Abstract]
Notes payable to Questar	273,300,000					273,300,000	119,300,000
Questar Gas [Member] | Estimated Fair Value [Member] | Level 2 [Member]
Financial liabilities [Abstract]
Long-term debt, including current portion	568,400,000					568,400,000	607,200,000
Wexpro [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Abandonment and impairment	12,100,000	$ 0	$ 0	$ 0		12,100,000
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, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Abandonment and impairment					$ 80,600,000	0	0	$ 80,600,000
Financial liabilities [Abstract]
Property, Plant, and Equipment, Fair Value Disclosure	24,000,000					24,000,000
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	10,200,000					10,200,000	7,400,000
Notes receivable from Questar	6,000,000					6,000,000	40,100,000
Financial liabilities [Abstract]
Long-term debt, including current portion	433,600,000					433,600,000	458,800,000
Questar Pipeline [Member] | Estimated Fair Value [Member] | Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	10,200,000					10,200,000	7,400,000
Notes receivable from Questar	6,000,000					6,000,000	40,100,000
Questar Pipeline [Member] | Estimated Fair Value [Member] | Level 2 [Member]
Financial liabilities [Abstract]
Long-term debt, including current portion	$ 445,200,000					$ 445,200,000	$ 495,500,000