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Strategic Review of Questar Southern Trails Pipeline (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Property, Plant and Equipment [Line Items]
Net book value of Questar Southern Trails Pipeline	$ 3,403.7	$ 3,317.0	$ 3,210.8
Questar Southern Trails Pipeline - Western Segment [Member]
Property, Plant and Equipment [Line Items]
Net book value of Questar Southern Trails Pipeline	21.0
Questar Southern Trails Pipeline - Eastern Segment [Member]
Property, Plant and Equipment [Line Items]
Net book value of Questar Southern Trails Pipeline	$ 81.0

Amount, net of accumulated depreciation, depletion and amortization, of long-lived physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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