Fair Value Measurements (Details) (USD $)	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
Carrying Amount [Member]
Financial assets [Abstract]
Cash and cash equivalents	$ 7,300,000	$ 16,000,000	$ 0
Long-term investment	15,800,000	17,300,000	16,800,000
Financial liabilities [Abstract]
Checks outstanding in excess of cash balances	0	0	1,600,000
Short-term debt	234,000,000	276,000,000	355,000,000
Long-term debt	1,245,700,000	1,247,100,000	1,097,500,000
Estimated Fair Value [Member] | Fair Value, Inputs, Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	7,300,000	16,000,000	0
Financial liabilities [Abstract]
Checks outstanding in excess of cash balances	0	0	1,600,000
Short-term debt	234,000,000	276,000,000	355,000,000
Estimated Fair Value [Member] | Fair Value, Inputs, Level 2 [Member]
Financial liabilities [Abstract]
Long-term debt	1,364,000,000	1,310,800,000	1,191,100,000
Estimated Fair Value [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Financial assets [Abstract]
Long-term investment	15,800,000	17,300,000	16,800,000
Questar Gas [Member] | Carrying Amount [Member]
Financial assets [Abstract]
Cash and cash equivalents	0	8,800,000	0
Financial liabilities [Abstract]
Checks outstanding in excess of cash balances	2,100,000	0	1,400,000
Notes payable to Questar	90,800,000	17,700,000	195,000,000
Long-term debt	534,500,000	534,500,000	384,500,000
Questar Gas [Member] | Estimated Fair Value [Member] | Fair Value, Inputs, Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	0	8,800,000	0
Financial liabilities [Abstract]
Checks outstanding in excess of cash balances	2,100,000	0	1,400,000
Notes payable to Questar	90,800,000	17,700,000	195,000,000
Questar Gas [Member] | Estimated Fair Value [Member] | Fair Value, Inputs, Level 2 [Member]
Financial liabilities [Abstract]
Long-term debt	602,800,000	568,000,000	425,600,000
Questar Pipeline [Member] | Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 3 [Member]
Financial liabilities [Abstract]
Property, Plant, and Equipment, Fair Value Disclosure			0.0
Questar Pipeline [Member] | Carrying Amount [Member]
Financial assets [Abstract]
Cash and cash equivalents	400,000	2,700,000	1,500,000
Notes receivable from Questar	35,200,000	29,400,000	36,600,000
Financial liabilities [Abstract]
Long-term debt	458,800,000	458,900,000	458,800,000
Questar Pipeline [Member] | Estimated Fair Value [Member] | Fair Value, Inputs, Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	400,000	2,700,000	1,500,000
Notes receivable from Questar	35,200,000	29,400,000	36,600,000
Questar Pipeline [Member] | Estimated Fair Value [Member] | Fair Value, Inputs, Level 2 [Member]
Financial liabilities [Abstract]
Long-term debt	$ 505,000,000	$ 486,000,000	$ 505,500,000