Mergers, Acquisitions, and Dispositions - Narrative (Details) - USD ($)		3 Months Ended							6 Months Ended
	Mar. 31, 2016	Mar. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Mar. 23, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Mar. 24, 2016	Dec. 31, 2015	Jul. 31, 2014
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Liabilities Noncurrent				$ 3,443,000,000	$ 3,443,000,000				$ 3,443,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets				1,441,000,000	1,441,000,000				1,441,000,000
Revenues				6,910,000,000				$ 6,514,000,000	14,485,000,000	$ 15,345,000,000
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Payments to Acquire Businesses, Gross									(6,642,000,000)	(28,000,000)
Business Combination, Integration Related Costs				1,000,000				(87,000,000)	103,000,000	21,000,000
Investments				$ 502,000,000	$ 502,000,000				$ 502,000,000			$ 639,000,000
Cash offered in exchange for each share of PHI stock				$ 27.25	$ 27.25				$ 27.25
Other noncurrent assets				$ 1,366,000,000	$ 1,366,000,000				$ 1,366,000,000			1,445,000,000
Other Commitment				6,399,000,000	6,399,000,000				6,399,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets				4,024,000,000	4,024,000,000				4,024,000,000
Business Combination, Contingent Consideration, Asset				235,000,000	235,000,000				235,000,000
Net income				306,000,000				633,000,000	430,000,000	1,372,000,000
Goodwill, Acquired During Period				8,000,000		$ 4,016,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Assets				248,000,000	248,000,000				248,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities				2,763,000,000	2,763,000,000				2,763,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt				5,636,000,000	5,636,000,000				5,636,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other				187,000,000	187,000,000				187,000,000
Common Stock [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Payments to Acquire Businesses, Gross							$ (180,000,000)
Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Liabilities Noncurrent				3,448,000,000	3,448,000,000				3,448,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets				1,441,000,000	1,441,000,000				1,441,000,000
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Assets				249,000,000	249,000,000				249,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities				2,764,000,000	2,764,000,000				2,764,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt				5,661,000,000	5,661,000,000				5,661,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other				187,000,000	187,000,000				187,000,000
Other Investments [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Other Commitment				130,000,000	130,000,000				130,000,000
Pepco Holdings LLC [Member]
Business Acquisition [Line Items]
Revenues				1,112,000,000					1,219,000,000
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Other Commitment				508,000,000	508,000,000				508,000,000
Net income				52,000,000					(262,000,000)
Pepco Holdings LLC [Member] | Common Stock [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Payments to Acquire Businesses, Gross									(6,933,000,000)
Pepco Holdings LLC [Member] | Common Stock [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Payments to Acquire Businesses, Gross									(180,000,000)
Pepco Holdings LLC [Member] | Successor [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Payments to Acquire Businesses, Gross					(62,000,000)
PEPCO Holdings Inc [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Payments to Acquire Businesses, Gross									(7,142,000,000)
Other Commitment				444,000,000	444,000,000				444,000,000
Fitzpatrick [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Payments to Acquire Businesses, Gross			$ (110,000,000)
Fitzpatrick [Member] | Maximum [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Business Combination, Integration Related Costs		$ 250,000,000
Exelon Generation Co L L C [Member]
Business Acquisition [Line Items]
Revenues				3,589,000,000				4,232,000,000	8,329,000,000	10,074,000,000
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Payments to Acquire Businesses, Gross									(1,000,000)	(28,000,000)
Business Combination, Integration Related Costs				4,000,000				7,000,000	20,000,000	15,000,000
Investments				$ 294,000,000	$ 294,000,000				$ 294,000,000			210,000,000
Equity Method Investment, Ownership Percentage				50.01%	50.01%				50.01%
Other noncurrent assets				$ 632,000,000	$ 632,000,000				$ 632,000,000			627,000,000
Gain (Loss) on Disposition of Property Plant Equipment									32,000,000
Other Commitment				5,589,000,000	5,589,000,000				5,589,000,000				$ 91,000,000
Business Combination, Contingent Consideration, Asset				163,000,000	163,000,000				163,000,000
Net income				28,000,000				390,000,000	285,000,000	875,000,000
Members' Capital				9,168,000,000	9,168,000,000				9,168,000,000			8,997,000,000
Atlantic City Electric Company [Member]
Business Acquisition [Line Items]
Revenues				270,000,000				285,000,000	561,000,000	618,000,000
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Business Combination, Integration Related Costs				2,000,000				1,000,000	15,000,000	1,000,000
Other noncurrent assets				23,000,000	23,000,000				23,000,000			19,000,000
Other Commitment				9,000,000	9,000,000				9,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets				1,098,000,000	1,098,000,000				1,098,000,000
Net income				3,000,000				6,000,000	(97,000,000)	15,000,000
Atlantic City Electric Company [Member] | Pepco Holdings LLC [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Other Commitment				120,000,000	120,000,000				120,000,000
Equity Contribution									49,000,000
Potomac Electric Power Company [Member]
Business Acquisition [Line Items]
Revenues				509,000,000				504,000,000	1,061,000,000	1,049,000,000
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Investments				99,000,000	99,000,000				99,000,000			68,000,000
Other noncurrent assets				4,000,000	4,000,000				4,000,000			4,000,000
Gain (Loss) on Disposition of Property Plant Equipment									8,000,000
Other Commitment				14,000,000	14,000,000				14,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets				1,691,000,000	1,691,000,000				1,691,000,000
Net income				49,000,000				42,000,000	(60,000,000)	68,000,000
Potomac Electric Power Company [Member] | Pepco Holdings LLC [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Other Commitment				139,000,000	139,000,000				139,000,000
Equity Contribution									73,000,000
Delmarva Power and Light Company [Member]
Business Acquisition [Line Items]
Revenues				281,000,000				271,000,000	643,000,000	691,000,000
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Business Combination, Integration Related Costs				0				1,000,000	16,000,000	1,000,000
Other noncurrent assets				8,000,000	8,000,000				8,000,000			$ 2,000,000
Other Commitment				11,000,000	11,000,000				11,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets				1,174,000,000	1,174,000,000				1,174,000,000
Net income				12,000,000				8,000,000	(60,000,000)	40,000,000
Delmarva Power and Light Company [Member] | Pepco Holdings LLC [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Other Commitment				104,000,000	104,000,000				104,000,000
Equity Contribution									46,000,000
Pepco Holdings LLC [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Business Combination, Integration Related Costs				(4,000,000)				$ 1,000,000	23,000,000	$ 2,000,000
Goodwill, Acquired During Period				8,000,000		$ 4,016,000,000
Pepco Holdings LLC [Member] | Successor [Member]
Business Acquisition [Line Items]
Revenues				1,066,000,000	1,171,000,000
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Business Combination, Integration Related Costs	$ 55,000,000			(1,000,000)
Investments				131,000,000	131,000,000				131,000,000
Other noncurrent assets				57,000,000	57,000,000				57,000,000
Business Combination, Contingent Consideration, Asset				251,000,000	251,000,000				251,000,000		$ (235,000,000)
Net income				52,000,000	(257,000,000)
Members' Capital				8,028,000,000	8,028,000,000				8,028,000,000		$ 7,200,000,000
Pepco Holdings LLC [Member] | Pepco Holdings LLC [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Other Commitment				363,000,000	363,000,000				363,000,000
Constellation Energy Group LLC [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Total liabilities held for sale (b)				5,000,000	5,000,000				5,000,000
Total assets held for sale				$ 61,000,000	$ 61,000,000				$ 61,000,000
Subsequent Event [Member] | ConEdision [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Payments to Acquire Businesses, Gross			(53,000,000)
Net Working Capital			$ 130,000,000
Subsequent Event [Member] | Fitzpatrick [Member] | Minimum [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Business Combination, Integration Related Costs		$ 200,000,000