Debt and Credit Agreements - Narrative (Details) $ / shares in Units, shares in Millions			3 Months Ended	12 Months Ended
		Sep. 30, 2013 MW	Sep. 30, 2014 USD ($)	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) $ / shares	Nov. 27, 2017 USD ($)	Mar. 31, 2017 USD ($)	Mar. 23, 2017 USD ($)	Nov. 30, 2016 USD ($)	Sep. 30, 2016 USD ($)	May 26, 2016 USD ($)	Apr. 01, 2016 USD ($)	Feb. 22, 2016	Jan. 13, 2016 USD ($)	Jan. 05, 2016 USD ($)	Oct. 23, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jul. 30, 2015	Jun. 30, 2015 USD ($)	Sep. 18, 2014 USD ($)	Jun. 30, 2014 USD ($) $ / shares shares	Dec. 31, 2013 USD ($)	Aug. 10, 2013 USD ($)	Jun. 30, 2013 USD ($)	Oct. 31, 2011 USD ($)	Aug. 01, 2011 USD ($)
Debt Instrument [Line Items]
Long-term Debt, Current Maturities				$ 2,088,000,000	$ 2,430,000,000
Repayments of Long-term Debt				2,490,000,000	1,936,000,000	$ 2,687,000,000
Line of credit facility, maximum borrowing capacity										$ 98
Outstanding letters of credit	[1]			1,147,000,000
Long-term debt, gross				33,657,000,000	33,311,000,000
Long-term debt				34,047,000,000
Collateral amount of debt				3,000,000,000
Outstanding borrowings/facility draws				$ 0
Credit agreement threshold				2.50
Senior Notes [Member]
Debt Instrument [Line Items]
Long-term debt, gross				$ 11,285,000,000	11,868,000,000
Junior Subordinated Debt [Member]
Debt Instrument [Line Items]
Long-term debt, gross				$ 1,150,000,000	1,150,000,000
Equity units issued | shares																						23
Conversion price | $ / shares																						$ 50
Interest rate on long-term debt				3.497%																		2.50%
Stock Issued During Period, Shares, Treasury Stock Reissued | shares				33
Junior Subordinated Notes																						$ 1,150,000,000
Loss on Reissuance, Treasury Stock				$ 1,050,000,000
Maximum [Member]
Debt Instrument [Line Items]
Basis points adders for prime-based borrowings				9000.00%
Basis points adders for LIBOR-based borrowings				165.0
Maximum [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate on long-term debt				7.60%
Maximum [Member] | Junior Subordinated Debt [Member]
Debt Instrument [Line Items]
Interest rate on long-term debt				3.50%
Minimum [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate on long-term debt				2.45%
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	[2]			$ 9,480,000,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity																								$ 500,000,000
Exelon Corporate [Member]
Debt Instrument [Line Items]
Repayments of Long-term Debt				569,000,000	46,000,000	2,263,000,000
Outstanding letters of credit	[1]			45,000,000
Long-term debt, gross				7,039,000,000	7,608,000,000
Long-term debt				7,039,000,000
Outstanding borrowings/facility draws				0
Short-term Bank Loans and Notes Payable									$ 500,000,000
Exelon Corporate [Member] | Junior Subordinated Debt [Member]
Debt Instrument [Line Items]
Long-term debt, gross				$ 1,150,000,000	1,150,000,000
Interest rate on long-term debt				3.50%
Exelon Corporate [Member] | Maximum [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate on long-term debt	[3]			7.60%
Exelon Corporate [Member] | Minimum [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate on long-term debt	[3]			2.45%
Exelon Corporate [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	[2]			$ 600,000,000
Exelon Generation Co L L C [Member]
Debt Instrument [Line Items]
Long-term Debt, Current Maturities				346,000,000	1,117,000,000
Repayments of Long-term Debt				1,261,000,000	202,000,000	89,000,000
Line of credit facility, maximum borrowing capacity				200,000,000
Long-term debt, gross				8,955,000,000	9,208,000,000
Long-term debt				$ 8,955,000,000
Short-term Debt, Percentage Bearing Variable Interest Rate														1.75%
Line of Credit Facility, Description				100
Credit agreement threshold				3.00
Proceeds from (Repayments of) Restricted Cash, Financing Activities				$ (50,000,000)
Exelon Generation Co L L C [Member] | ExGen Texas Power [Member] | Interest Rate Swap [Member]
Debt Instrument [Line Items]
Interest rate on long-term debt																					2.34%
Non-recourse debt, interest rate swap			$ 505,000,000
Exelon Generation Co L L C [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Long-term debt, gross				6,019,000,000	5,971,000,000
Debt Instrument, Face Amount				750,000,000
Exelon Generation Co L L C [Member] | Non Recourse Debt [Member] | ExGen Texas Power [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount																					$ 675,000,000
Exelon Generation Co L L C [Member] | Non Recourse Debt [Member] | DOE Project Financing, 3.092% January 2, 2037 [Member]
Debt Instrument [Line Items]
Outstanding letters of credit				105,000,000
Non-recourse debt, commitment																							$ 646,000,000
Debt, Weighted Average Interest Rate																							2.82%
Non-recourse debt				530,000,000
Exelon Generation Co L L C [Member] | Non Recourse Debt [Member] | Denver International Airport [Member]
Debt Instrument [Line Items]
Interest rate on long-term debt																									5.50%
Outstanding borrowings/facility draws																									$ 7,000,000
Non-recourse debt				6,000,000
Exelon Generation Co L L C [Member] | Non Recourse Debt [Member] | Holyoke [Member]
Debt Instrument [Line Items]
Interest rate on long-term debt																										5.25%
Outstanding borrowings/facility draws																										$ 11,000,000
Non-recourse debt				9,000,000
Exelon Generation Co L L C [Member] | Non Recourse Debt [Member] | Upstream Gas Property [Member]
Debt Instrument [Line Items]
Repayments of Long-term Debt				31,000,000	15,000,000
Escrow Deposit				6,000,000
Gain (Loss) on Sale of Debt Investments				37,000,000
Long-term debt				15,000,000
Outstanding borrowings/facility draws				22,000,000
Proceeds from Sales of Business, Affiliate and Productive Assets				$ 1,000,000
Exelon Generation Co L L C [Member] | Non Recourse Debt [Member] | SolGen [Member]
Debt Instrument [Line Items]
Interest rate on long-term debt				3.93%
Debt Instrument, Face Amount																					$ 150,000,000
Non-recourse debt				$ 147,000,000
Exelon Generation Co L L C [Member] | Non Recourse Debt [Member] | Renewable Power Generation [Member]
Debt Instrument [Line Items]
Interest rate on long-term debt								4.11%
Debt Instrument, Face Amount								$ 150,000,000
Non-recourse debt				127,000,000
Exelon Generation Co L L C [Member] | Non Recourse Debt [Member] | Continetal Wind [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity											$ 131,000,000
Outstanding letters of credit				114,000,000
Interest rate on long-term debt																		6.00%
Debt Instrument, Face Amount																		$ 613,000,000
Generation capacity of portfolio | MW		667
Aggregate bank commitments under unsecured revolving credit facilities																		$ 10,000,000
Non-recourse debt				512,000,000
Exelon Generation Co L L C [Member] | Non Recourse Debt [Member] | ExGenRenewablesIVNov2024 [Member]
Debt Instrument [Line Items]
Interest rate on long-term debt							3.00%
Long-term debt							$ 850,000,000
Non-recourse debt, interest rate swap				$ 636,000,000
Non-recourse debt, hedge percentage				2.32%
Proceeds from Issuance of Debt				$ 785,000,000
Proceeds from (Repayments of) Restricted Cash, Financing Activities				$ 50,000,000
Exelon Generation Co L L C [Member] | Maximum [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate on long-term debt				7.60%
Exelon Generation Co L L C [Member] | Minimum [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate on long-term debt				2.95%
Exelon Generation Co L L C [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity				$ 5,300,000,000
Outstanding letters of credit				868,000,000
Outstanding borrowings/facility draws				0
Exelon Generation Co L L C [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	[2]			480,000,000
Outstanding letters of credit	[1]			231,000,000
Outstanding borrowings/facility draws				0
Exelon Generation Co L L C [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity													$ 200,000,000			$ 150,000,000	$ 100,000,000
Exelon Generation Co L L C [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Outstanding letters of credit				480,000,000	500,000,000
Commonwealth Edison Co [Member]
Debt Instrument [Line Items]
Long-term Debt, Current Maturities				840,000,000	425,000,000
Repayments of Long-term Debt				425,000,000	665,000,000	260,000,000
Outstanding letters of credit	[1]			2,000,000
Long-term debt, gross				7,676,000,000	7,101,000,000
Long-term debt				7,882,000,000
Outstanding borrowings/facility draws				$ 0
Credit agreement threshold				2.00
Commonwealth Edison Co [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	[2]			$ 1,000,000,000
PECO Energy Co [Member]
Debt Instrument [Line Items]
Repayments of Long-term Debt				0	300,000,000	0
Outstanding letters of credit	[1]			1,000,000
Long-term debt, gross				2,925,000,000	2,600,000,000
Long-term debt				3,109,000,000
Outstanding borrowings/facility draws				$ 0
Credit agreement threshold				2.00
PECO Energy Co [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	[2]			$ 600,000,000
Baltimore Gas and Electric Company [Member]
Debt Instrument [Line Items]
Repayments of Long-term Debt				41,000,000	379,000,000	$ 75,000,000
Outstanding letters of credit	[1]			0
Long-term debt, gross				2,600,000,000	2,341,000,000
Long-term debt				2,600,000,000
Outstanding borrowings/facility draws				$ 0
Preferred Stock, Redemption Price Per Share | $ / shares						$ 100
Credit agreement threshold				2.00
Baltimore Gas and Electric Company [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Long-term debt, gross				$ 2,600,000,000	2,300,000,000
Baltimore Gas and Electric Company [Member] | Maximum [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate on long-term debt				6.35%
Baltimore Gas and Electric Company [Member] | Minimum [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate on long-term debt				2.40%
Baltimore Gas and Electric Company [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	[2]			$ 600,000,000
Pepco Holdings LLC [Member]
Debt Instrument [Line Items]
Long-term debt, gross				5,189,000,000	5,157,000,000
Long-term debt				5,189,000,000
Repayments of Bank Debt				300,000,000
Short-term Bank Loans and Notes Payable															$ 500,000,000					$ 300,000,000
Short-term Debt, Percentage Bearing Variable Interest Rate															1.00%				950000.00%
Pepco Holdings LLC [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity												$ 900,000,000															$ 1,500,000,000
Potomac Electric Power Company [Member]
Debt Instrument [Line Items]
Repayments of Long-term Debt				13,000,000	11,000,000	$ 22,000,000
Outstanding letters of credit	[1]			0
Long-term debt, gross				2,570,000,000	2,381,000,000
Long-term debt				2,570,000,000
Outstanding borrowings/facility draws				$ 0
Credit agreement threshold				2.00
Potomac Electric Power Company [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	[2]			$ 300,000,000
Delmarva Power and Light Company [Member]
Debt Instrument [Line Items]
Long-term Debt, Current Maturities				79,000,000	105,000,000
Repayments of Long-term Debt				40,000,000	100,000,000	100,000,000
Outstanding letters of credit	[1]			0
Long-term debt, gross				1,309,000,000	1,348,000,000
Long-term debt				1,309,000,000
Outstanding borrowings/facility draws				$ 0
Credit agreement threshold				2.00
Delmarva Power and Light Company [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	[2]			$ 300,000,000
Atlantic City Electric Company [Member]
Debt Instrument [Line Items]
Repayments of Long-term Debt				35,000,000	48,000,000	$ 58,000,000
Outstanding letters of credit	[1]			0
Long-term debt, gross				1,127,000,000	$ 1,162,000,000
Long-term debt				1,127,000,000
Outstanding borrowings/facility draws				$ 0
Credit agreement threshold				2.00
Atlantic City Electric Company [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	[2]			$ 300,000,000

[1] Excludes nonrecourse debt letters of credit, see discussion below on Antelope Valley Solar Ranch One and Continental Win

[2] Excludes additional credit facility agreements for Generation, ComEd, PECO, BGE, Pepco, DPL and ACE with aggregate commitments of $49 million, $34 million, $34 million, $5 million, $2 million, $2 million and $2 million, respectively, arranged with minority and community banks located primarily within utilities' service territories. These facilities expire on October 12, 2018. These facilities are solely utilized to issue letters of credit. As of December 31, 2017, letters of credit issued under these facilities totaled $5 million and $2 million for Generation and BGE, respectively.

[3] Senior unsecured notes include mirror debt that is held on both Generation and Exelon Corporation's balance sheets.