Fair Value Measurements (Details 2) (USD $) In Millions	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Fair Value Assets and Liabilities Measured on Recurring Basis Unobservable Input Reconciliation Calculation
Balance, Level 3 fair value measurements at beginning of period	$ (172.8)	$ (315.2)	$ (201.2)	$ (291.5)
Realized and unrealized (losses) gains recorded in income	39.8	58.0	(20.5)	(78.7)
Realized and unrealized (losses) gains recorded in other comprehensive income	19.2	(2.8)	29.5	73.6
Purchase	(3.6)		(3.6)
Issuances	(3.5)		1.1
Net purchases, sales, issuances, and settlements	(7.1)	7.4	(2.5)	16.6
Transfers into Level 3	34.1	93.0	145.7	208.0
Transfers out of Level 3	46.7	(16.1)	8.9	(103.7)
Balance, Level 3 fair value measurements at end of period	(40.1)	(175.7)	(40.1)	(175.7)
Change in unrealized gains recorded in income relating to derivatives still held at the end of the period	$ 23.0	$ 82.9	$ (1.4)	$ 8.9

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

Name:	ceg_FairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward
Namespace Prefix:	ceg
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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

Name:	ceg_FairValueAssetsLiabilitiesMeasuredOnRecurringBasisChangeInUnrealizedGainLossIncludedInEarnings
Namespace Prefix:	ceg
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

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

+ Details

Name:	ceg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues
Namespace Prefix:	ceg
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

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

+ Details

Name:	ceg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
Namespace Prefix:	ceg
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This element represents total 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
Namespace Prefix:	ceg
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This element represents total 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
Namespace Prefix:	ceg
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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
Namespace Prefix:	ceg
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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
Namespace Prefix:	ceg
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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
Namespace Prefix:	ceg
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- 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

Name:	ceg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisNetAssetLiabilityValue
Namespace Prefix:	ceg
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant