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Common Stock Plans (Stock Options, Range of Exercise Prices) (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Number of exercisable options, Weighted Average Remaining Contractual Life (in years) 5 years 9 months 15 days
Stock options | Stock Incentive Plan  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Number of outstanding options, Range of Exercise Prices, Lower Range Limit 29.79us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit
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Number of outstanding options, Range of Exercise Prices, Upper Range Limit 80.48us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit
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Number of outstanding options, Shares 757,071us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions
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Number of outstanding options, Weighted Average Remaining Contractual Life (in years) 6 years 8 months 5 days
Stock options | Stock Incentive Plan | $46.45  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Number of outstanding options, Range of Exercise Prices 46.45egn_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRange
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Number of outstanding options, Shares 25,470us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions
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= egn_ExercisePriceRange46.45Member
Number of outstanding options, Weighted Average Remaining Contractual Life (in years) 2 years
Stock options | Stock Incentive Plan | $60.56  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Number of outstanding options, Range of Exercise Prices 60.56egn_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRange
/ us-gaap_AwardTypeAxis
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Number of outstanding options, Shares 48,560us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions
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= egn_ExercisePriceRange60.56Member
Number of outstanding options, Weighted Average Remaining Contractual Life (in years) 3 years
Stock options | Stock Incentive Plan | $29.79  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Number of outstanding options, Range of Exercise Prices 29.79egn_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRange
/ us-gaap_AwardTypeAxis
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/ us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis
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Number of outstanding options, Shares 35,161us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions
/ us-gaap_AwardTypeAxis
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/ us-gaap_PlanNameAxis
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/ us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis
= egn_ExercisePriceRange29.79Member
Number of outstanding options, Weighted Average Remaining Contractual Life (in years) 4 years
Stock options | Stock Incentive Plan | $46.69  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Number of outstanding options, Range of Exercise Prices 46.69egn_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRange
/ us-gaap_AwardTypeAxis
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Number of outstanding options, Shares 26,481us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions
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= egn_ExercisePriceRange46.69Member
Number of outstanding options, Weighted Average Remaining Contractual Life (in years) 5 years
Stock options | Stock Incentive Plan | $54.99  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Number of outstanding options, Range of Exercise Prices 54.99egn_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRange
/ us-gaap_AwardTypeAxis
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Number of outstanding options, Shares 111,487us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions
/ us-gaap_AwardTypeAxis
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/ us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis
= egn_ExercisePriceRange54.99Member
Number of outstanding options, Weighted Average Remaining Contractual Life (in years) 6 years
Stock options | Stock Incentive Plan | $54.11  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Number of outstanding options, Range of Exercise Prices 54.11egn_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRange
/ us-gaap_AwardTypeAxis
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Number of outstanding options, Shares 271,848us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions
/ us-gaap_AwardTypeAxis
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= egn_ExercisePriceRange54.11Member
Number of outstanding options, Weighted Average Remaining Contractual Life (in years) 7 years
Stock options | Stock Incentive Plan | $48.36  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Number of outstanding options, Range of Exercise Prices 48.36egn_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRange
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Number of outstanding options, Shares 124,071us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions
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= egn_ExercisePriceRange48.36Member
Number of outstanding options, Weighted Average Remaining Contractual Life (in years) 8 years
Stock options | Stock Incentive Plan | $80.48  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Number of outstanding options, Range of Exercise Prices 80.48egn_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRange
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Number of outstanding options, Shares 3,686us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions
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= egn_ExercisePriceRange80.48Member
Number of outstanding options, Weighted Average Remaining Contractual Life (in years) 8 years 9 months 15 days
Stock options | Stock Incentive Plan | $72.39  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Number of outstanding options, Range of Exercise Prices 72.39egn_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRange
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Number of outstanding options, Shares 107,868us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions
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= egn_ExercisePriceRange72.39Member
Number of outstanding options, Weighted Average Remaining Contractual Life (in years) 9 years
Stock options | Stock Incentive Plan | $79.63  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Number of outstanding options, Range of Exercise Prices 79.63egn_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRange
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Number of outstanding options, Shares 2,439us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions
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Number of outstanding options, Weighted Average Remaining Contractual Life (in years) 9 years