Fair Value Measurement (Schedule Of Valuation Processes) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012
Minimum [Member] | Natural Gas Liquids [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Forward Curve Range	3.90
Maximum [Member] | Natural Gas Liquids [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Forward Curve Range	4.39
Natural Gas Liquids [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Assets	$ 49.1
Natural Gas Liquids [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Forward Curve Range	3.74
Natural Gas Liquids [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Forward Curve Range	4.39
Natural Gas [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Assets	1.4
Fair Value, Liabilities	$ 0.4
Natural Gas [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Forward Curve Range	0.34
Natural Gas [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Forward Curve Range	2.02