Derivative Financial Instruments - Summary of Proprietary Trading Activities (Details) - Consolidation Items [Domain] - Entity [Domain] - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	$ (2)	$ (6)
Operating Revenue [Member]
Derivative [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net			$ 1	$ (7)
Operating Revenue [Member] \| Principal or Proprietary Transactions [Member]
Derivative [Line Items]
Proprietary Trading Activities Change In Fair Value	0	0	4	(1)
Proprietary Trading Activities Reclassification To Realized At Settlement	(2)	1	(6)	1
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(2)	1	(2)	0
Commodity Contract [Member] \| Operating Revenue [Member] \| Principal or Proprietary Transactions [Member]
Derivative [Line Items]
Proprietary Trading Activities Change In Fair Value	7	1	8	0
Proprietary Trading Activities Reclassification To Realized At Settlement	(7)	(8)	(5)	(7)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	$ 0	$ (7)	$ 3	$ (7)

Represents the pre-tax mark-to-market gains or losses of certain purchase and sale contracts due to the change in fair value of the derivative contracts held at the reporting date. These contracts are entered into purely to profit from market prices changes as opposed to hedging an exposure.

No definition available.

+ Details

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Namespace Prefix:	exc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the pre-tax mark-to-market gains or losses of certain purchase and sale contracts due to the recognized change in fair value that was reclassified to realized due to settlement of the derivative during the period. These contracts are entered into purely to profit from market prices changes as opposed to hedging an exposure.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4C

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624171-113959

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Details

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- Details

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