Mergers, Acquisitions, and Dispositions Purchase Price Allocation Table (Details) $ in Millions	Jun. 30, 2016 USD ($)
Business Acquisition [Line Items]
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	11,076
BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedRegulatedAssets	5,015
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Assets	248
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	21,804
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities	2,763
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Financial Liabilities	1,515
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, Noncurrent Liabilities, Other	187
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	14,662
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	4,024
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Liabilities Noncurrent	3,443
_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedRetirmentObligations	821
Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets	1,441
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Assets	249
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities	2,764
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt	5,661
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other	187
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Liabilities Noncurrent	3,448
Potomac Electric Power Company [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets	1,691
Delmarva Power and Light Company [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets	$ 1,174