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
		Mar. 23, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Loss			$ 1,056	$ 413	$ (46)
Total before tax			3,756	1,989	3,330
Income Tax Expense (Benefit)			(125)	761	1,073
Net Income (Loss)			3,849	1,204	2,250
Amounts reclassified from AOCI	[1]		(144)	(150)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Loss	[1]			(5)
Total before tax	[1]			(5)
Income Tax Expense (Benefit)	[1]			0
Net Income (Loss)	[1]			(5)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Cash Flow Hedging [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Income Tax Expense (Benefit)			(1)	(5)
Gains and (Losses) on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Amounts reclassified from AOCI	[2],[3]		(4)	(8)
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 on Derivatives [Line Items]
Other cash flow hedges	[1]		(5)	(13)
Total before tax	[1]		(5)	(13)
Income Tax Expense (Benefit)	[1]		1	5
Net of tax	[1]		(4)	(8)
Prior service costs
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Amortization of pension and other postretirement benefit plan items	[1],[4]		92	78
Actuarial losses
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Amortization of pension and other postretirement benefit plan items	[1],[4]		(324)	(302)
Pension and Non-Pension Postretirement Benefit Plan Items
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Amortization of pension and other postretirement benefit plan items	[1]		(232)	(224)
Tax benefit	[1]		92	87
Amounts reclassified from AOCI	[1],[2],[3]		(140)	(137)
Exelon Generation Co L L C [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Loss			948	401	(60)
Income Tax Expense (Benefit)			(1,375)	290	502
Net Income (Loss)			2,771	558	1,340
Amounts reclassified from AOCI	[1]		(4)	(13)
Exelon Generation Co L L C [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Loss	[1]			(5)
Total before tax	[1]			(5)
Income Tax Expense (Benefit)	[1]			0
Net Income (Loss)	[1]			(5)
Exelon Generation Co L L C [Member] | Gains and (Losses) on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Amounts reclassified from AOCI	[2],[3]		(4)	(8)
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 on Derivatives [Line Items]
Other cash flow hedges	[1]		(5)	(13)
Total before tax	[1]		(5)	(13)
Income Tax Expense (Benefit)	[1]		1	5
Net of tax	[1]		(4)	(8)
Exelon Generation Co L L C [Member] | Prior service costs
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [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 on Derivatives [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 on Derivatives [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] | Predecessor [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Loss		$ (4)			88
Income Tax Expense (Benefit)		17			163
Net of tax		19			318
Net Income (Loss)		19			$ 327
Amounts reclassified from AOCI	[1]	(1)
Pepco Holdings LLC [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Predecessor [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Loss	[1]	0
Total before tax	[1]	0
Income Tax Expense (Benefit)	[1]	0
Net Income (Loss)	[1]	0
Pepco Holdings LLC [Member] | Gains and (Losses) on Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income [Member] | Cash Flow Hedging [Member] | Predecessor [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other cash flow hedges	[1]	0
Total before tax	[1]	0
Income Tax Expense (Benefit)	[1]	0
Net of tax	[1]	0
Pepco Holdings LLC [Member] | Prior service costs | Predecessor [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Amortization of pension and other postretirement benefit plan items	[1],[4]	0
Pepco Holdings LLC [Member] | Actuarial losses | Predecessor [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Amortization of pension and other postretirement benefit plan items	[1],[4]	(1)
Pepco Holdings LLC [Member] | Pension and Non-Pension Postretirement Benefit Plan Items | Predecessor [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Amortization of pension and other postretirement benefit plan items	[1]	(1)
Tax benefit	[1]	0
Amounts reclassified from AOCI	[1]	$ (1)

[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 details).