Significant Risks and Uncertainties (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended			12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012	Dec. 31, 2012 February 2011 West Storage Incident [Member]	Dec. 31, 2011 February 2011 West Storage Incident [Member]	Dec. 31, 2012 BP [Member]
Revenue, Major Customer [Line Items]
Entity-Wide Revenue, Major Customer, Percentage						9.50%
Insurance Matters [Abstract]
Minimum insurance deductible per incident			$ 5.0
Maximum insurance deductible per incident			60.0
Coverage by certain producers for each of the Independence Hub and Marco Polo platforms			350.0
Schedule of Gains Recognized From Business Insurance Recoveries [Line Items]
Gains recognized from business interruption settlements	4.3	1.1
Loss Contingencies [Line Items]
Estimate of cost related to incident				150
Gains related to property damage proceeds		56.6		30.0	4.7
Proceeds from property damage insurance recoveries		$ 64.8		$ 30.0	$ 20.0