Fair Value Measurements (Details 2) (Recurring Measurements, USD $) In Millions	3 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Recurring Measurements
Fair Value Assets and Liabilities Measured on Recurring Basis Unobservable Input Reconciliation Calculation
Balance, Level 3 fair value measurements at beginning of period	$ (40.1)	$ (175.7)	$ (201.2)	$ (291.5)
Realized and unrealized (losses) gains recorded in income	(50.3)	(145.8)	(70.7)	(203.4)
Realized and unrealized (losses) gains recorded in other comprehensive income	(10.4)	14.1	19.1	87.6
Purchases	0.6		(3.0)
Issuances	2.3		3.4
Net purchases, sales, issuances, and settlements	2.9	28.9	0.4	24.5
Transfers into Level 3	(30.4)	(105.3)	115.3	102.7
Transfers out of Level 3	33.5	(41.1)	42.3	(144.8)
Balance, Level 3 fair value measurements at end of period	(94.8)	(424.9)	(94.8)	(424.9)
Change in unrealized gains recorded in income relating to derivatives still held at the end of the period	$ 44.4	$ (75.3)	$ (19.4)	$ (96.9)

Represents a roll forward of assets and liabilities reconciliation of a concept from the beginning of a period to the end of a period.

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- Definition

This item represents the amount of the total change in unrealized (holding) gains or losses for the period which are included in the statement of income. Such unrealized (holding) gains or losses relate to those assets and liabilities still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3).

+ Details

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- Definition

This element represents total realized and unrealized gains or losses for the period, arising from net 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.

+ Details

Name:	ceg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisNetAssetLiabilityGainLossIncludedInEarnings
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Balance Type:	debit
Period Type:	duration

- Definition

This element represents total realized and unrealized gains or losses for the period, arising from net assets and liabilities measured at fair value on a recurring basis using unobservable inputs (Level 3), which are included in other comprehensive income (a separate component of shareholders' equity).

+ Details

Name:	ceg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisNetAssetLiabilityGainLossIncludedInOtherComprehensiveIncome
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- Definition

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

+ Details

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Balance Type:	debit
Period Type:	duration

- Definition

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

+ Details

Name:	ceg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisNetAssetLiabilityPurchases
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Balance Type:	debit
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- Definition

This element represents purchases, sales, issuances, and settlements (net) which have taken place during the period in relation to net assets and liabilities measured at fair value on a recurring basis using unobservable inputs (Level 3).

+ Details

Name:	ceg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisNetAssetLiabilityPurchasesSalesIssuancesSettlements
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Period Type:	duration

- Definition

This element represents transfers into assets and liabilities measured at fair value on a recurring basis using unobservable inputs (Level 3) which have taken place during the period.

+ Details

Name:	ceg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisNetAssetLiabilityTransfersIn
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- Definition

This element represents transfers out of assets and liabilities measured at fair value on a recurring basis using unobservable inputs (Level 3) which have taken place during the period.

+ Details

Name:	ceg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisNetAssetLiabilityTransfersOut
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- Definition

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

+ Details

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