Derivative Commodity Instruments - Derivative Instruments Change in Fair Value (Details) (Derivative Commodity Instruments, USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2012	Mar. 31, 2011
Derivative Commodity Instruments
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	$ 65,801	$ 42,755
Realized losses	(1,291)	(702)
Unrealized gains (losses) relating to instruments held at the reporting date	54,239	(9,870)
Settlements during period	(13,826)	(15,256)
Balance at end of period	$ 104,923	$ 16,927

This element represents unrealized gains or losses for the period arising from assets and liabilities measured at fair value on a recurring basis using unobservable inputs (Level 3), which are included in earnings or resulted in a change in net asset value.

No definition available.

Name:	egn_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilityGainLossIncludedInEarningsUnrealized
Namespace Prefix:	egn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration