Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Pension and other postretirement benefits	$ (202.8)	$ (202.8)	$ (104.1)
Interest rate cash flow hedges	(23.2)	(23.4)	(22.8)
Long-term investment	0.1	0.1	0.1
Accumulated other comprehensive (loss)	(225.9)	(226.1)	(126.8)
Questar Pipeline [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Interest rate cash flow hedges	(23.2)	(23.4)	(22.8)
Accumulated other comprehensive (loss)	$ (23.2)	$ (23.4)	$ (22.8)

Accumulated appreciation or loss, net of tax, in value of the total of available-for-sale securities at the end of an accounting period.

+ References+ Details

Accumulated change, net of tax, in accumulated gains and losses from derivative instruments designated and qualifying as the effective portion of cash flow hedges. Includes an entity's share of an equity investee's Increase or Decrease in deferred hedging gains or losses.

+ References+ Details

- Definition

The total of net gain (loss), prior service cost (credit), and transition assets (obligations), as well as minimum pension liability if still remaining, included in accumulated other comprehensive income associated with a defined benefit pension or other postretirement plan(s) because they have yet to be recognized as components of net periodic benefit cost.

+ References

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+ Details

- Definition

Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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+ Details