Mergers, Acquisitions, and Dispositions Purchase Price Allocation Table (Details) $ in Millions	Mar. 31, 2016 USD ($)
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets	$ 2,107
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment	11,071
BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedRegulatedAssets	5,118
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Assets	656
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	22,968
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities	3,425
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Financial Liabilities	1,550
BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedRegulatedLiabilities	297
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt	6,076
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other	191
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	15,826
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	7,142
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets	4,016
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Liabilities Noncurrent	3,441
_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedRetirmentObligations	846
Potomac Electric Power Company [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets	1,695
Delmarva Power and Light Company [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets	$ 1,182