Derivative Instruments and Hedging Activities Asset and Liability Derivative Instruments Designated as Hedges in the Balance Sheets (Details) (Interest Rate Derivative Instruments [Member], Level 2 [Member], USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011
Derivatives, Fair Value [Line Items]
Consolidated total - derivative assets	$ 0	$ 8.9	$ 4.9
Consolidated total - derivative liabilities	0	0	0.2
Fair Value Hedging [Member] | Prepaid Expenses and Other [Member]
Derivatives, Fair Value [Line Items]
Interest rate derivative instruments, current assets	0	1.1	0.7
Fair Value Hedging [Member] | Other Noncurrent Assets [Member]
Derivatives, Fair Value [Line Items]
Interest rate derivative instrument, noncurrent assets	0	7.8	3.2
Questar Pipeline [Member] | Cash Flow Hedging [Member] | Prepaid Expenses and Other [Member]
Derivatives, Fair Value [Line Items]
Interest rate derivative instruments, current assets	0	0	1.0
Questar Pipeline [Member] | Cash Flow Hedging [Member] | Accounts Payable and Accrued Expenses [Member]
Derivatives, Fair Value [Line Items]
Interest rate derivative instruments, current liabilities	$ 0	$ 0	$ 0.2