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
		Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
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		$ 880	$ 1,319	$ 949	$ 1,003
Total realized / unrealized gains (losses)
Included in net income		(228)	186
Included in noncurrent payables to affiliates		0	0
Included in regulatory assets		20	(4)
Change in collateral		81	105
Purchases, sales, issuances and settlements
Purchases		58	90
Sales		0	(3)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances			0
Settlements		(17)	(48)
Transfers into Level 3		0	(8)
Transfers out of Level 3		17	(2)
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		(148)	258
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
Total realized / unrealized gains (losses)
Included in net income		0	0
Included in noncurrent payables to affiliates		(11)	(7)
Included in regulatory assets		11	7
Change in collateral		0
Purchases, sales, issuances and settlements
Purchases		0	0
Sales		0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances			0
Settlements		0	0
Transfers into Level 3		0	0
Transfers out of Level 3		0	0
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		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		80	72
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		1,081	1,563	1,160	1,237
Total realized / unrealized gains (losses)
Included in net income		(229)	185
Included in noncurrent payables to affiliates		11	7
Included in regulatory assets		0	0
Change in collateral		81	105
Purchases, sales, issuances and settlements
Purchases		58	90
Sales		0	(3)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances			0
Settlements		(17)	(48)
Transfers into Level 3		0	(8)
Transfers out of Level 3		17	(2)
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		(149)	257
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		540	609	543	648
Total realized / unrealized gains (losses)
Included in net income		2	0
Included in noncurrent payables to affiliates		11	7
Included in regulatory assets			0
Change in collateral		0	0
Purchases, sales, issuances and settlements
Purchases		1	2
Sales		0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances			0
Settlements		(17)	(48)
Transfers into Level 3		0	0
Transfers out of Level 3		0	0
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		2	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		499	918	575	552
Total realized / unrealized gains (losses)
Included in net income	[1]	(231)	184
Included in noncurrent payables to affiliates		0	0
Included in regulatory assets			0
Change in collateral		81	105
Purchases, sales, issuances and settlements
Purchases		57	88
Sales		0	(3)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances			0
Settlements		0	0
Transfers into Level 3	[2]	0	(8)
Transfers out of Level 3	[2]	17	0
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		(151)	256
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		42	36	42	37
Total realized / unrealized gains (losses)
Included in net income		0	1
Included in noncurrent payables to affiliates		0	0
Included in regulatory assets			0
Change in collateral		0	0
Purchases, sales, issuances and settlements
Purchases		0	0
Sales		0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances			0
Settlements		0	0
Transfers into Level 3		0	0
Transfers out of Level 3		0	(2)
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		0	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)		(5)	(6)
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		(240)	(267)	(249)	(256)
Total realized / unrealized gains (losses)
Included in net income		0	0
Included in noncurrent payables to affiliates		0	0
Included in regulatory assets	[3]	9	11
Change in collateral		0	0
Purchases, sales, issuances and settlements
Purchases		0	0
Sales		0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances			0
Settlements		0	0
Transfers into Level 3		0	0
Transfers out of Level 3		0	0
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		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)		(14)	17
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	[4]	39	23	$ 38	$ 22
Total realized / unrealized gains (losses)
Included in net income	[4]	1	1
Included in noncurrent payables to affiliates	[4]	0	0
Included in regulatory assets	[4]	0	0
Change in collateral	[4]	0	0
Purchases, sales, issuances and settlements
Purchases	[4]	0	0
Sales	[4]	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances	[4]		0
Settlements	[4]	0	0
Transfers into Level 3	[4]	0	0
Transfers out of Level 3	[4]	0	0
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	[4]	$ 1	$ 1

[1] Includes a reduction for the reclassification of $80 million and $72 million of realized gains due to the settlement of derivative contracts for the three months ended March 31, 2019 and 2018, respectively.

[2] Transfers into and out of Level 3 generally occur when the contract tenor becomes less and more observable respectively, primarily due to changes in market liquidity or assumptions for certain commodity contracts.

[3] Includes $14 million of decreases in fair value and an increase for realized losses due to settlements of $5 million recorded in purchased power expense associated with floating-to-fixed energy swap contracts with unaffiliated suppliers for the three months ended March 31, 2019. Includes $17 million of increases in fair value and an increase for realized losses due to settlements of $6 million recorded in purchased power expense associated with floating-to-fixed energy swap contracts with unaffiliated suppliers for the three months ended March 31, 2018.

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