Mergers, Acquisitions and Dispositions Purchase Price Allocation Table - (PHI) (Details) - Pepco Holdings LLC [Member] $ in Millions	Mar. 23, 2016 USD ($)
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	$ 21,797
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities	2,752
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets	1,441
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment	11,088
BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedRegulatedAssets	5,015
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Assets	248
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets	4,005
BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedRegulatedLiabilities	297
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt	5,636
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Liabilities Noncurrent	3,447
_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedRetirmentObligations	821
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other	187
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	14,655
BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedEnergyContract	1,515
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	$ 7,142