Mergers, Acquisitions, and Dispositions - Narrative (Details) - USD ($)		1 Months Ended	3 Months Ended					6 Months Ended	9 Months Ended		12 Months Ended
	Mar. 31, 2016	Jul. 31, 2016	Mar. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Mar. 23, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Aug. 31, 2016	Mar. 24, 2016	Jul. 31, 2014
Business Acquisition [Line Items]
Revenues				$ 9,002,000,000			$ 7,401,000,000		$ 23,486,000,000	$ 22,746,000,000
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Payments to Acquire Businesses, Gross									(6,896,000,000)	(28,000,000)
Business Combination, Integration Related Costs				20,000,000			22,000,000		123,000,000	84,000,000
Investments				$ 592,000,000				$ 592,000,000	$ 592,000,000		$ 639,000,000
Cash offered in exchange for each share of PHI stock				$ 27.25				$ 27.25	$ 27.25
Other noncurrent assets				$ 1,474,000,000				$ 1,474,000,000	$ 1,474,000,000		1,445,000,000
Other Commitment	$ 508,000,000			6,497,000,000				6,497,000,000	6,497,000,000			$ 513,000,000
Business Combination, Contingent Consideration, Asset				235,000,000				235,000,000	235,000,000
Net income				526,000,000			587,000,000		956,000,000	1,959,000,000
Goodwill, Acquired During Period						$ (4,016,000,000)			16,000,000
Net Working Capital									96,000,000
Other Intangible Assets [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Finite-lived Intangible Assets Acquired									1,500,000,000
Common Stock [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Payments to Acquire Businesses, Gross											(180,000,000)
Other Investments [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Other Commitment				137,000,000				137,000,000	137,000,000
Other Investments [Member] | Scenario, Adjustment [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Other Commitment				55,000,000				55,000,000	55,000,000
Pepco Holdings LLC [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Liabilities Noncurrent				3,442,000,000				3,442,000,000	3,442,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets				1,441,000,000				1,441,000,000	1,441,000,000
Revenues				1,437,000,000					2,656,000,000
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Other Commitment				513,000,000				513,000,000	513,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets				4,000,000,000				4,000,000,000	4,000,000,000
Net income				169,000,000					(92,000,000)
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				2,752,000,000				2,752,000,000	2,752,000,000
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
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment				11,088,000,000				11,088,000,000	11,088,000,000
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					$ (33,000,000)			(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)
PEPCO Holdings Inc [Member] | Other Investments [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Other Commitment				444,000,000				444,000,000	444,000,000
ConEdision [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Business Combination, Consideration Transferred, Liabilities Incurred									22,000,000
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				5,035,000,000			4,768,000,000		13,363,000,000	14,841,000,000
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Payments to Acquire Businesses, Gross									(255,000,000)	(28,000,000)
Business Combination, Integration Related Costs				9,000,000			10,000,000		29,000,000	30,000,000
Investments				$ 381,000,000				$ 381,000,000	$ 381,000,000		210,000,000
Equity Method Investment, Ownership Percentage				50.01%				50.01%	50.01%
Other noncurrent assets				$ 692,000,000				$ 692,000,000	$ 692,000,000		627,000,000
Gain (Loss) on Disposition of Property Plant Equipment									32,000,000
Other Commitment				5,679,000,000				5,679,000,000	5,679,000,000					$ 91,000,000
Business Combination, Contingent Consideration, Asset				163,000,000				163,000,000	163,000,000
Net income				271,000,000			332,000,000		556,000,000	1,208,000,000
Members' Capital				9,265,000,000				9,265,000,000	9,265,000,000		8,997,000,000
Exelon Generation Co L L C [Member] | ConEdision [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Payments to Acquire Businesses, Gross				(257,000,000)					(235,000,000)
Net Working Capital				204,000,000
Atlantic City Electric Company [Member]
Business Acquisition [Line Items]
Revenues				421,000,000			386,000,000		982,000,000	1,003,000,000
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Business Combination, Integration Related Costs				2,000,000			0		17,000,000	1,000,000
Other noncurrent assets				42,000,000				42,000,000	42,000,000		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				1,158,000,000				1,158,000,000	1,158,000,000
Net income				47,000,000			22,000,000		(50,000,000)	37,000,000
Atlantic City Electric Company [Member] | Other Investments [Member] | Scenario, Adjustment [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Other Commitment				10,000,000				10,000,000	10,000,000
Atlantic City Electric Company [Member] | Pepco Holdings LLC [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Other Commitment				111,000,000				111,000,000	111,000,000
Equity Contribution									49,000,000
Potomac Electric Power Company [Member]
Business Acquisition [Line Items]
Revenues				635,000,000			592,000,000		1,695,000,000	1,641,000,000
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Investments				100,000,000				100,000,000	100,000,000		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				15,000,000				15,000,000	15,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets				1,683,000,000				1,683,000,000	1,683,000,000
Net income				79,000,000			60,000,000		20,000,000	128,000,000
Potomac Electric Power Company [Member] | Successor [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Payments to Acquire Businesses, Gross									(33,000,000)
Potomac Electric Power Company [Member] | Other Investments [Member] | Scenario, Adjustment [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Other Commitment				13,000,000				13,000,000	13,000,000
Potomac Electric Power Company [Member] | Pepco Holdings LLC [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Other Commitment				126,000,000				126,000,000	126,000,000
Equity Contribution									73,000,000
Delmarva Power and Light Company [Member]
Business Acquisition [Line Items]
Revenues				331,000,000			314,000,000		974,000,000	1,004,000,000
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Business Combination, Integration Related Costs				2,000,000			0		18,000,000	2,000,000
Other noncurrent assets				7,000,000				7,000,000	7,000,000		$ 2,000,000
Gain (Loss) on Disposition of Property Plant Equipment		$ 4,000,000
Other Commitment				11,000,000				11,000,000	11,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets				1,128,000,000				1,128,000,000	1,128,000,000
Net income				44,000,000			15,000,000		(16,000,000)	55,000,000
Delmarva Power and Light Company [Member] | Other Investments [Member] | Scenario, Adjustment [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Other Commitment				27,000,000				27,000,000	27,000,000
Delmarva Power and Light Company [Member] | Pepco Holdings LLC [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Other Commitment				77,000,000				77,000,000	77,000,000
Equity Contribution									46,000,000
Pepco Holdings LLC [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Business Combination, Integration Related Costs				3,000,000			$ 1,000,000		26,000,000	$ 3,000,000
Goodwill, Acquired During Period						$ (4,016,000,000)
Pepco Holdings LLC [Member] | Scenario, Adjustment [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Fresh-Start Adjustment, Increase (Decrease), Deferred Income Tax Liabilities, Noncurrent													$ 59,000,000
Pepco Holdings LLC [Member] | Successor [Member]
Business Acquisition [Line Items]
Revenues				1,394,000,000				2,565,000,000
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Business Combination, Integration Related Costs	$ 63,000,000			7,000,000
Investments				132,000,000				132,000,000	132,000,000
Other noncurrent assets				76,000,000				76,000,000	76,000,000
Business Combination, Contingent Consideration, Asset													301,000,000
Net income				166,000,000				(91,000,000)
Members' Capital				7,978,000,000				7,978,000,000	7,978,000,000				$ 7,200,000,000
Pepco Holdings LLC [Member] | Other Investments [Member] | Scenario, Adjustment [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Other Commitment				50,000,000				50,000,000	50,000,000
Pepco Holdings LLC [Member] | Pepco Holdings LLC [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Other Commitment				314,000,000				314,000,000	314,000,000
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
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment				$ 46,000,000				$ 46,000,000	$ 46,000,000
Subsequent Event [Member] | Fitzpatrick [Member] | Minimum [Member]
Payments to Acquire Businesses and Interest in Affiliates [Abstract]
Business Combination, Integration Related Costs			$ 200,000,000