Fair Value Measurements - Summary of Changes of Derivative Commodity Instruments in Fair Value (Details) (Derivative Commodity Instruments, USD $)	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Derivative Commodity Instruments
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ 24,436,000	$ 18,289,000
Realized gains (losses)	13,153,000	(3,019,000)
Unrealized losses relating to instruments held at the reporting date	(22,023,000)	(16,835,000)
Settlements during period	(13,153,000)	2,943,000
Balance at end of period	2,413,000	1,378,000
Mark-to-market loss included in earnings, unrealized	$ 10,300,000	$ 13,200,000

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This element represents realized 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.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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- Definition

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.

+ References

No definition available.

+ Details

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

- Definition

Settlements that have taken place during the period in relation to assets and liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy.

+ References

No definition available.

+ Details

Name:	egn_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilityPurchasesAndSettlements
Namespace Prefix:	egn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

This element represents an asset and liability measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances.

+ References

No definition available.

+ Details

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Namespace Prefix:	egn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of open derivatives, commodity, or energy contracts, held at each balance sheet date, that was included in earnings for the period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585

+ Details

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Period Type:	duration