Schedule of Valuation and Qualifying Accounts (Details) - Allowance for Bad Debt [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning Balance	$ 1.7	$ 1.7	$ 3.1
Amounts charged to expense	2.1	1.7	0.2
Deductions for accounts written off and other	(1.7)	(1.7)	(1.6)
Ending Balance	2.1	1.7	1.7
Questar Gas [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning Balance	1.4	1.4	2.8
Amounts charged to expense	2.1	1.7	0.2
Deductions for accounts written off and other	(1.8)	(1.7)	(1.6)
Ending Balance	1.7	1.4	1.4
Questar Pipeline [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning Balance	0.3	0.3	0.3
Amounts charged to expense	0.0	0.0	0.0
Deductions for accounts written off and other	0.1	0.0	0.0
Ending Balance	$ 0.4	$ 0.3	$ 0.3