Changes in Accumulated Other Comprehensive Income Changes in Accumulated Other Comprehensive Income - Reclassifications out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
		Dec. 31, 2018	Mar. 23, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss				$ (112)	$ 947	$ 297
Income Tax Expense (Benefit)		$ 26		120	(126)	753
Net income (loss)				2,084	3,876	1,196
Amortization of pension and other postretirement benefit plan items				4
Amounts reclassified from AOCI	[1]			(182)	(144)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Cash Flow Hedging [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income Tax Expense (Benefit)					(1)
Gains and (Losses) on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI	[2],[3]			(1)	(4)
Gains and (Losses) on Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income [Member] | Cash Flow Hedging [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other cash flow hedges	[1]			(1)	(5)
Total before tax	[1]			(1)	(5)
Income Tax Expense (Benefit)	[1]			0	1
Net of tax	[1]			(1)	(4)
Prior service costs
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of pension and other postretirement benefit plan items	[1],[4]			90	92
Actuarial losses
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of pension and other postretirement benefit plan items	[1],[4]			(333)	(324)
Pension and Non-Pension Postretirement Benefit Plan Items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of pension and other postretirement benefit plan items	[1]			(243)	(232)
Tax benefit	[1]			62	92
Amounts reclassified from AOCI	[1],[2],[3]			(181)	(140)
Exelon Generation Co L L C [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss				(178)	948	401
Income Tax Expense (Benefit)				(108)	(1,376)	282
Net income (loss)				443	2,798	$ 550
Amortization of pension and other postretirement benefit plan items				3
Amounts reclassified from AOCI	[1]			(1)	(4)
Exelon Generation Co L L C [Member] | Gains and (Losses) on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI	[2],[3]			(1)	(4)
Exelon Generation Co L L C [Member] | Gains and (Losses) on Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income [Member] | Cash Flow Hedging [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other cash flow hedges	[1]			(1)	(5)
Total before tax	[1]			(1)	(5)
Income Tax Expense (Benefit)	[1]			0	1
Net of tax	[1]			(1)	(4)
Exelon Generation Co L L C [Member] | Prior service costs
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of pension and other postretirement benefit plan items	[1],[4]			0	0
Exelon Generation Co L L C [Member] | Actuarial losses
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of pension and other postretirement benefit plan items	[1],[4]			0	0
Exelon Generation Co L L C [Member] | Pension and Non-Pension Postretirement Benefit Plan Items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of pension and other postretirement benefit plan items	[1]			0	0
Tax benefit	[1]			0	0
Amounts reclassified from AOCI	[1],[2],[3]			0	0
Pepco Holdings LLC [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss				43	54
Income Tax Expense (Benefit)				35	217
Net income (loss)				$ 398	$ 362
Pepco Holdings LLC [Member] | Predecessor [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss			$ (4)
Income Tax Expense (Benefit)			17
Net income (loss)			$ 19

[1] Amounts in parenthesis represent a decrease in net income.

[2] All amounts are net of tax and noncontrolling interests. Amounts in parenthesis represent a decrease in AOCI.

[3] See next tables for details about these reclassifications.

[4] This AOCI component is included in the computation of net periodic pension and OPEB cost. See Note 16 — Retirement Benefits for additional information.