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				9 Months Ended
		Sep. 30, 2018		Sep. 30, 2017		Sep. 30, 2018		Sep. 30, 2017		Jun. 30, 2018		Dec. 31, 2017		Jun. 30, 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		$ 908		$ 1,070		$ 908		$ 1,070		$ 1,160		$ 1,015		$ 1,098		993
Total realized / unrealized gains (losses)
Included in net income		(246)		(80)		(172)		(102)
Included in noncurrent payables to affiliates		0		0		0		0
Included in payable for Zion Station decommissioning		2		(4)		7		(3)
Included in regulatory assets		(11)		(21)		(3)		(6)
Change in collateral		(44)		11		14		81
Purchases, sales, issuances and settlements
Purchases		99		77		219		205
Sales		(20)		0		(23)		(15)
Issuances								(1)
Settlements		(29)		(21)		(103)		(85)
Transfers into Level 3		3		0		(21)		(9)
Transfers out of Level 3		(6)		10		(25)		12
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		(92)		26		168		167
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
Total realized / unrealized gains (losses)
Included in net income		0				0		0
Included in noncurrent payables to affiliates		4				0		(13)
Included in payable for Zion Station decommissioning		0				0		0
Included in regulatory assets		(4)				0		13
Change in collateral		0				0		0
Purchases, sales, issuances and settlements
Purchases		0				0		0
Sales		0				0		0
Issuances								0
Settlements		0				0		0
Transfers into Level 3		0				0		0
Transfers out of Level 3		0				0		0
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		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		(155)		96		(347)		279
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		1,130		1,326		1,130		1,326		1,376		1,249		1,334		1,231
Total realized / unrealized gains (losses)
Included in net income		(247)		(81)		(175)		(104)
Included in noncurrent payables to affiliates		(4)		0		0		13
Included in payable for Zion Station decommissioning		2		(4)		7		(3)
Included in regulatory assets		0		0		0		0
Change in collateral		(44)		11		14		81
Purchases, sales, issuances and settlements
Purchases		99		77		219		205
Sales		(20)		0		(23)		(15)
Issuances								0
Settlements		(29)		(21)		(115)		(85)
Transfers into Level 3		3		0		(21)		(9)
Transfers out of Level 3		(6)		10		(25)		12
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		(92)		25		168		165
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		566		671		566		671		585		648		683		677
Total realized / unrealized gains (losses)
Included in net income		(1)		0		(1)		4
Included in noncurrent payables to affiliates		(4)		0		0		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		15		19		34		54
Sales		0		0		0		0
Issuances								0
Settlements		(29)		(31)		(115)		(77)
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)		(1)		0		(5)		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		0		17		0		17		18		12		21		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		(4)		7		(3)
Included in regulatory assets				0				0
Change in collateral		0		0		0		0
Purchases, sales, issuances and settlements
Purchases		0		0		1		1
Sales		(20)		0		(20)		0
Issuances								0
Settlements		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		512		595		512		595		737		552		589		493
Total realized / unrealized gains (losses)
Included in net income		(259)	[1]	(82)	[2]	(188)	[1]	(110)	[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		(44)		11		14		81
Purchases, sales, issuances and settlements
Purchases		81		57		181		146
Sales		0		0		(3)		(15)
Issuances								0
Settlements		0		10		0		(8)
Transfers into Level 3		3		0		(21)		(9)
Transfers out of Level 3		(6)		10		(23)		17
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		(104)		24		159		161
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		52		43		52		43		36		37		41		42
Total realized / unrealized gains (losses)
Included in net income		13		1		14		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		0		0		0		0
Purchases, sales, issuances and settlements
Purchases		3		1		3		4
Sales		0		0		0		0
Issuances								0
Settlements		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)		13		1		14		2
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)		(3)		(3)		(12)		(13)
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		(259)		(277)		(259)		(277)		(252)		(256)		(256)		(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		(7)	[3]	21	[4]	(3)	[3]	(19)	[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)		4		24		(9)		32
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		37	[5]	21	[6]	37	[5]	21	[6]	$ 36	[5]	$ 22	[5]	$ 20	[6]	$ 20	[6]
Total realized / unrealized gains (losses)
Included in net income		(1)	[5]	(1)	[6]	(3)	[5]	(2)	[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		0	[5]	0	[6]	(12)	[5]	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]	$ 1	[6]	$ 0	[5]	$ 2	[6]

[1] Includes a reduction for the reclassification of $155 million and $347 million of realized losses due to the settlement of derivative contracts for the three and nine months ended September 30, 2018, respectively.

[2] Includes a reduction for the reclassification of $96 million and $279 million of realized gains due to the settlement of derivative contracts for the three and nine months ended September 30, 2017, respectively.

[3] Includes $4 million of increases in fair value and an increase for realized losses due to settlements of $3 million recorded in purchased power expense associated with floating-to-fixed energy swap contracts with unaffiliated suppliers for the three months ended September 30, 2018. Includes $9 million of decreases in fair value and an increase for realized losses due to settlements of $12 million recorded in purchased power expense associated with floating-to-fixed energy swap contracts with unaffiliated suppliers for the nine months ended September 30, 2018.

[4] Includes $24 million of increases in fair value and an increase for realized losses due to settlements of $3 million recorded in purchased power expense associated with floating-to-fixed energy swap contracts with unaffiliated suppliers for the three months ended September 30, 2017. Includes $32 million of decreases in fair value and an increase for realized losses due to settlements of $13 million recorded in purchased power expense associated with floating-to-fixed energy swap contracts with unaffiliated suppliers for the nine months ended September 30, 2017.

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

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