Fair Value of Financial Assets and Liabilities - Fair Value Assets Liabilities Measured On Recurring Basis Unobservable Input Reconciliation (Details) - USD ($) $ in Millions		3 Months Ended				6 Months Ended
		Jun. 30, 2018		Jun. 30, 2017		Jun. 30, 2018		Jun. 30, 2017		Mar. 31, 2018		Dec. 31, 2017		Mar. 31, 2017		Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value																$ 0
Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		$ 1,160		$ 1,098		$ 1,160		$ 1,098		$ 1,335		$ 1,015		$ 1,046		993
Total realized / unrealized gains (losses)
Included in net income		(112)		(2)		75		(40)
Included in noncurrent payables to affiliates		0		0		0		0
Included in payable for Zion Station decommissioning		2		1		5		1
Included in regulatory assets		12		30		7		15
Change in collateral		(49)		31		57		69
Purchases, sales, issuances and settlements
Purchases		30		41		120		130
Sales		(1)		(13)		(4)		(15)
Issuances								(1)
Settlements		(26)		(24)		(74)		(47)
Transfers into Level 3		(15)		(8)		(23)		(10)
Transfers out of Level 3		(16)		(4)		(18)		3
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		3		43		260		106
Fair Value, Inputs, Level 3 [Member] | Consolidation, Eliminations [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		0		0		0		0				0		0
Total realized / unrealized gains (losses)
Included in net income		0		0		0		0
Included in noncurrent payables to affiliates		3		(4)		(3)		(13)
Included in payable for Zion Station decommissioning		0				0		0
Included in regulatory assets		(3)		4		3		13
Change in collateral		0				0		0
Purchases, sales, issuances and settlements
Purchases		0		0		0		0
Sales		0		0		0		0
Issuances								0
Settlements		0		0		0		0
Transfers into Level 3		0		0		0		0
Transfers out of Level 3		0		0		0		0
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		0		0		0		0
Exelon Generation Co L L C [Member]
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Gain (Loss) Included in Earnings		(120)		(46)		(192)		(148)
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		1,376		1,334		1,376		1,334		1,579		1,249		1,308		1,231
Total realized / unrealized gains (losses)
Included in net income		(113)		(2)		73		(41)
Included in noncurrent payables to affiliates		(3)		4		3		13
Included in payable for Zion Station decommissioning		2		1		5		1
Included in regulatory assets		0		0		0		0
Change in collateral		(49)		31		57		69
Purchases, sales, issuances and settlements
Purchases		30		41		120		130
Sales		(1)		(13)		(4)		(15)
Issuances								0
Settlements		(38)		(24)		(86)		(47)
Transfers into Level 3		(15)		(8)		(23)		(10)
Transfers out of Level 3		(16)		(4)		(18)		3
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		3		43		260		105
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Nuclear Decommissioning Trust Fund Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		585		683		585		683		609		648		683		677
Total realized / unrealized gains (losses)
Included in net income		0		1		1		4
Included in noncurrent payables to affiliates		(3)		4		3		13
Included in payable for Zion Station decommissioning		0		0		0		0
Included in regulatory assets				0				0
Change in collateral		0		0		0		0
Purchases, sales, issuances and settlements
Purchases		17		19		19		36
Sales		0		0		0		0
Issuances								0
Settlements		(38)		(24)		(86)		(47)
Transfers into Level 3		0		0		0		0
Transfers out of Level 3		0		0		0		0
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		(4)		0		(4)		2
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Pledged Assets For Zion Station Decommissioning [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		18		21		18		21		16		12		20		19
Total realized / unrealized gains (losses)
Included in net income		0		0		0		0
Included in noncurrent payables to affiliates		0		0		0		0
Included in payable for Zion Station decommissioning		2		1		5		1
Included in regulatory assets				0				0
Change in collateral		0		0		0		0
Purchases, sales, issuances and settlements
Purchases		0		0		1		1
Sales		0		0		0		0
Issuances								0
Settlements		0		0		0		0
Transfers into Level 3		0		0		0		0
Transfers out of Level 3		0		0		0		0
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		0		0		0		0
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		737		589		737		589		918		552		565		493
Total realized / unrealized gains (losses)
Included in net income		(113)	[1]	(3)	[2]	71	[1]	(46)	[2]
Included in noncurrent payables to affiliates		0		0		0		0
Included in payable for Zion Station decommissioning		0		0		0		0
Included in regulatory assets				0				0
Change in collateral		(49)		31		57		69
Purchases, sales, issuances and settlements
Purchases		13		21		100		90
Sales		(1)		(13)		(4)		(15)
Issuances								0
Settlements		0		0		0		0
Transfers into Level 3		(15)		(8)		(23)		(10)
Transfers out of Level 3		(16)		(4)		(16)		8
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		7		43		263		102
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Other Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		36		41		36		41		36		37		40		42
Total realized / unrealized gains (losses)
Included in net income		0		0		1		1
Included in noncurrent payables to affiliates		0		0		0		0
Included in payable for Zion Station decommissioning		0		0		0		0
Included in regulatory assets				0				0
Change in collateral		0		0		0		0
Purchases, sales, issuances and settlements
Purchases		0		1		0		3
Sales		0		0		0		0
Issuances								0
Settlements		0		0		0		0
Transfers into Level 3		0		0		0		0
Transfers out of Level 3		0		0		(2)		(5)
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		0		0		1		1
Commonwealth Edison Co [Member] | Derivative [Member] | Interest Rate Swap [Member]
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Realized Investment Gains (Losses)		(4)		(1)		(10)		(7)
Commonwealth Edison Co [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		(252)		(256)		(252)		(256)		(267)		(256)		(282)		(258)
Total realized / unrealized gains (losses)
Included in net income		0		0		0		0
Included in noncurrent payables to affiliates		0		0		0		0
Included in payable for Zion Station decommissioning		0		0		0		0
Included in regulatory assets		15	[3]	(26)	[4]	4	[3]	2	[4]
Change in collateral		0		0		0		0
Purchases, sales, issuances and settlements
Purchases		0		0		0		0
Sales		0		0		0		0
Issuances								0
Settlements		0		0		0		0
Transfers into Level 3		0		0		0		0
Transfers out of Level 3		0		0		0		0
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		0		0		0		0
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Period Increase (Decrease)		11		25		(6)		5
Pepco Holdings LLC [Member] | Fair Value, Inputs, Level 3 [Member] | 6311 Life Insurance [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		36	[5]	20	[6]	36	[5]	20	[6]	$ 23	[5]	$ 22	[5]	$ 20	[6]	$ 20	[6]
Total realized / unrealized gains (losses)
Included in net income		(1)	[5]	0	[6]	(2)	[5]	(1)	[6]
Included in noncurrent payables to affiliates		0	[5]	0	[6]	0	[5]	0	[6]
Included in payable for Zion Station decommissioning		0	[5]	0	[6]	0	[5]	0	[6]
Included in regulatory assets		0	[5]	0	[6]	0	[5]	0	[6]
Change in collateral		0	[5]	0	[6]	0	[5]	0
Purchases, sales, issuances and settlements
Purchases		0	[5]	0	[6]	0	[5]	0	[6]
Sales		0	[5]	0	[6]	0	[5]	0	[6]
Issuances	[6]							(1)
Settlements		(12)	[5],[7]	0	[6]	(12)	[5],[7]	0	[6]
Transfers into Level 3		0	[5]	0	[6]	0	[5]	0	[6]
Transfers out of Level 3		0	[5]	0	[6]	0	[5]	0	[6]
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		$ 0	[5]	$ 0	[6]	$ 0	[5]	$ 1	[6]

[1] Includes a reduction for the reclassification of $120 million and $192 million of realized gains due to the settlement of derivative contracts for the three and six months ended June 30, 2018, respectively.

[2] Includes a reduction for the reclassification of $46 million and $148 million of realized gains due to the settlement of derivative contracts for the three and six months ended June 30, 2017, respectively.

[3] Includes $11 million of increases in fair value and an increase for realized losses due to settlements of $4 million recorded in purchased power expense associated with floating-to-fixed energy swap contracts with unaffiliated suppliers for the three months ended June 30, 2018. Includes $6 million of decreases in fair value and an increase for realized losses due to settlements of $10 million recorded in purchased power expense associated with floating-to-fixed energy swap contracts with unaffiliated suppliers for the six months ended June 30, 2018.

[4] Includes $25 million of increases in fair value and an increase for realized losses due to settlements of $1 million recorded in purchased power expense associated with floating-to-fixed energy swap contracts with unaffiliated suppliers for the three months ended June 30, 2017. Includes $5 million of decreases in fair value and an increase for realized losses due to settlements of $7 million recorded in purchased power expense associated with floating-to-fixed energy swap contracts with unaffiliated suppliers for the six months ended June 30, 2017.

[5] The amounts represented are life insurance contracts at Pepco.

[6] The amounts represented are life insurance contracts at Pepco.

[7] The settlement amount represents the full payment of a loan held against one of Pepco's life insurance policy contracts.