Supplemental Financial Information - Cash Flow (Details) - USD ($) $ in Millions			3 Months Ended
		Feb. 28, 2018	Mar. 31, 2018	Mar. 31, 2017
Depreciation, Amortization and Accretion [Abstract]
Depreciation	[1]		$ 926	$ 754
Regulatory assets	[1]		152	128
Amortization of intangible assets, net	[1]		13	14
Amortization of Power Contracts Emission Credits	[2]		3	2
Amortization of Nuclear Fuel Lease	[3]		287	264
Asset Retirement Obligation, Accretion Expense	[4]		120	112
Total depreciation, amortization and accretion			1,501	1,274
Other Non-Cash Operating Activities [Abstract]
Pension and Other Postretirement Benefits Cost (Reversal of Cost)			145	157
Gain (loss) on equity method investments			7	10
Provision for uncollectible accounts			64	34
Stock-based compensation costs			29	31
Other Decommissioning Related Activity	[5]		(31)	(84)
Energy-related options	[6]		(7)	(4)
Amortization of regulatory asset related to debt costs			2	2
Amortization of rate stabilization deferral			7	(14)
Amortization of debt fair value adjustment			(3)	(5)
Discrete impacts from EIMA	[7]		(4)	(24)
Amortization of debt costs			9	9
Inventory Write-down			13	2
Other			9	4
Total other noncash operating activities			240	118
Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract]
Capital Expenditures Incurred but Not yet Paid			(177)	(193)
Payments to Acquire Businesses, Gross			0	212
Capitalized Costs, Asset Retirement Costs			32
Noncash or Part Noncash Acquisition, Fixed Assets Acquired				10
Stock Issued			1	2
Exelon Generation Co L L C [Member]
Depreciation, Amortization and Accretion [Abstract]
Depreciation	[1]		436	289
Regulatory assets	[1]		0	0
Amortization of intangible assets, net	[1]		12	13
Amortization of Power Contracts Emission Credits	[2]		3	2
Amortization of Nuclear Fuel Lease	[3]		287	264
Asset Retirement Obligation, Accretion Expense	[4]		120	110
Total depreciation, amortization and accretion			858	678
Other Non-Cash Operating Activities [Abstract]
Pension and Other Postretirement Benefits Cost (Reversal of Cost)			51	54
Gain (loss) on equity method investments			7	10
Provision for uncollectible accounts			11	9
Stock-based compensation costs			0	0
Other Decommissioning Related Activity	[5]		(31)	(84)
Energy-related options	[6]		(7)	(4)
Amortization of regulatory asset related to debt costs			0	0
Amortization of rate stabilization deferral			0	0
Amortization of debt fair value adjustment			(3)	(3)
Discrete impacts from EIMA			0	0
Amortization of debt costs			3	4
Inventory Write-down			12	1
Other			2	3
Total other noncash operating activities			45	(10)
Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract]
Capital Expenditures Incurred but Not yet Paid			(131)	(56)
Payments to Acquire Businesses, Gross		$ 87	0	212
Capitalized Costs, Asset Retirement Costs			32
Noncash or Part Noncash Acquisition, Fixed Assets Acquired				10
Stock Issued			0	0
Commonwealth Edison Co [Member]
Depreciation, Amortization and Accretion [Abstract]
Depreciation	[1]		201	190
Regulatory assets	[1]		27	18
Amortization of intangible assets, net	[1]		0	0
Amortization of Power Contracts Emission Credits	[2]		0	0
Amortization of Nuclear Fuel Lease	[3]		0	0
Asset Retirement Obligation, Accretion Expense	[4]		0	0
Total depreciation, amortization and accretion			228	208
Other Non-Cash Operating Activities [Abstract]
Pension and Other Postretirement Benefits Cost (Reversal of Cost)			45	44
Gain (loss) on equity method investments			0	0
Provision for uncollectible accounts			8	7
Stock-based compensation costs			0	0
Other Decommissioning Related Activity			0	0
Energy-related options			0	0
Amortization of regulatory asset related to debt costs			1	1
Amortization of rate stabilization deferral			0	0
Amortization of debt fair value adjustment			0	0
Discrete impacts from EIMA	[7]		(4)	(24)
Amortization of debt costs			1	1
Inventory Write-down			1	1
Other			(6)	1
Total other noncash operating activities			46	31
Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract]
Capital Expenditures Incurred but Not yet Paid			(48)	(66)
Capitalized Costs, Asset Retirement Costs			0
Noncash or Part Noncash Acquisition, Fixed Assets Acquired				0
Stock Issued			0	0
PECO Energy Co [Member]
Depreciation, Amortization and Accretion [Abstract]
Depreciation	[1]		68	64
Regulatory assets	[1]		7	7
Amortization of intangible assets, net	[1]		0	0
Amortization of Power Contracts Emission Credits	[2]		0	0
Amortization of Nuclear Fuel Lease	[3]		0	0
Asset Retirement Obligation, Accretion Expense	[4]		0	0
Total depreciation, amortization and accretion			75	71
Other Non-Cash Operating Activities [Abstract]
Pension and Other Postretirement Benefits Cost (Reversal of Cost)			5	7
Gain (loss) on equity method investments			0	0
Provision for uncollectible accounts			17	17
Stock-based compensation costs			0	0
Other Decommissioning Related Activity			0	0
Energy-related options			0	0
Amortization of regulatory asset related to debt costs			0	0
Amortization of rate stabilization deferral			0	0
Amortization of debt fair value adjustment			0	0
Discrete impacts from EIMA			0	0
Amortization of debt costs			0	0
Inventory Write-down			0	0
Other			(1)	(1)
Total other noncash operating activities			21	23
Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract]
Capital Expenditures Incurred but Not yet Paid			(25)	(42)
Capitalized Costs, Asset Retirement Costs			0
Noncash or Part Noncash Acquisition, Fixed Assets Acquired				0
Stock Issued			0	0
Baltimore Gas and Electric Company [Member]
Depreciation, Amortization and Accretion [Abstract]
Depreciation	[1]		82	80
Regulatory assets	[1]		52	48
Amortization of intangible assets, net	[1]		0	0
Amortization of Power Contracts Emission Credits	[2]		0	0
Amortization of Nuclear Fuel Lease	[3]		0	0
Asset Retirement Obligation, Accretion Expense			0	0	[4]
Total depreciation, amortization and accretion			134	128
Other Non-Cash Operating Activities [Abstract]
Pension and Other Postretirement Benefits Cost (Reversal of Cost)			14	16
Gain (loss) on equity method investments			0	0
Provision for uncollectible accounts			8	5
Stock-based compensation costs			0	0
Other Decommissioning Related Activity			0	0
Energy-related options			0	0
Amortization of regulatory asset related to debt costs			0	0
Amortization of rate stabilization deferral			0	7
Amortization of debt fair value adjustment			0	0
Discrete impacts from EIMA			0	0
Amortization of debt costs			0	0
Inventory Write-down			0	0
Other			(2)	(4)
Total other noncash operating activities			20	24
Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract]
Capital Expenditures Incurred but Not yet Paid			(11)	1
Capitalized Costs, Asset Retirement Costs			0
Noncash or Part Noncash Acquisition, Fixed Assets Acquired				0
Stock Issued			0	0
Potomac Electric Power Company [Member]
Depreciation, Amortization and Accretion [Abstract]
Depreciation	[1]		53	50
Regulatory assets	[1]		43	32
Amortization of intangible assets, net	[1]		0	0
Amortization of Power Contracts Emission Credits	[2]		0	0
Amortization of Nuclear Fuel Lease	[3]		0	0
Asset Retirement Obligation, Accretion Expense	[4]		0	0
Total depreciation, amortization and accretion			96	82
Other Non-Cash Operating Activities [Abstract]
Pension and Other Postretirement Benefits Cost (Reversal of Cost)			4	7
Gain (loss) on equity method investments			0	0
Provision for uncollectible accounts			6	(5)
Stock-based compensation costs			0	0
Other Decommissioning Related Activity			0	0
Energy-related options			0	0
Amortization of regulatory asset related to debt costs			0	0
Amortization of rate stabilization deferral			1	(15)
Amortization of debt fair value adjustment			0	0
Discrete impacts from EIMA			0	0
Amortization of debt costs			0	0
Inventory Write-down			0	0
Other			(1)	(2)
Total other noncash operating activities			10	(15)
Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract]
Capital Expenditures Incurred but Not yet Paid			(19)	(6)
Capitalized Costs, Asset Retirement Costs			0
Noncash or Part Noncash Acquisition, Fixed Assets Acquired				0
Stock Issued			0	0
Delmarva Power and Light Company [Member]
Depreciation, Amortization and Accretion [Abstract]
Depreciation	[1]		32	30
Regulatory assets	[1]		13	9
Amortization of intangible assets, net	[1]		0	0
Amortization of Power Contracts Emission Credits	[2]		0	0
Amortization of Nuclear Fuel Lease	[3]		0	0
Asset Retirement Obligation, Accretion Expense	[4]		0	0
Total depreciation, amortization and accretion			45	39
Other Non-Cash Operating Activities [Abstract]
Pension and Other Postretirement Benefits Cost (Reversal of Cost)			0	3
Gain (loss) on equity method investments			0	0
Provision for uncollectible accounts			8	(1)
Stock-based compensation costs			0	0
Other Decommissioning Related Activity			0	0
Energy-related options			0	0
Amortization of regulatory asset related to debt costs			0	0
Amortization of rate stabilization deferral			6	(6)
Amortization of debt fair value adjustment			0	0
Discrete impacts from EIMA			0	0
Amortization of debt costs			0	0
Inventory Write-down			0	0
Other			5	(3)
Total other noncash operating activities			19	(7)
Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract]
Capital Expenditures Incurred but Not yet Paid			14	9
Capitalized Costs, Asset Retirement Costs			0
Noncash or Part Noncash Acquisition, Fixed Assets Acquired				0
Stock Issued			0	0
Atlantic City Electric Company [Member]
Depreciation, Amortization and Accretion [Abstract]
Depreciation	[1]		23	21
Regulatory assets	[1]		10	14
Amortization of intangible assets, net	[1]		0	0
Amortization of Power Contracts Emission Credits	[2]		0	0
Amortization of Nuclear Fuel Lease	[3]		0	0
Asset Retirement Obligation, Accretion Expense	[4]		0	0
Total depreciation, amortization and accretion			33	35
Other Non-Cash Operating Activities [Abstract]
Pension and Other Postretirement Benefits Cost (Reversal of Cost)			3	3
Gain (loss) on equity method investments			0	0
Provision for uncollectible accounts			5	1
Stock-based compensation costs			0	0
Other Decommissioning Related Activity			0	0
Energy-related options			0	0
Amortization of regulatory asset related to debt costs			0	0
Amortization of rate stabilization deferral			0	0
Amortization of debt fair value adjustment			0	0
Discrete impacts from EIMA			0	0
Amortization of debt costs			0	0
Inventory Write-down			0	0
Other			1	(2)
Total other noncash operating activities			9	2
Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract]
Capital Expenditures Incurred but Not yet Paid			27	0
Capitalized Costs, Asset Retirement Costs			0
Noncash or Part Noncash Acquisition, Fixed Assets Acquired				0
Stock Issued			0	0
Pepco Holdings LLC [Member]
Depreciation, Amortization and Accretion [Abstract]
Depreciation			117	112
Regulatory assets			66	55
Amortization of intangible assets, net			0	0
Amortization of Power Contracts Emission Credits			0	0
Amortization of Nuclear Fuel Lease			0	0
Asset Retirement Obligation, Accretion Expense			0	0
Total depreciation, amortization and accretion			183	167
Other Non-Cash Operating Activities [Abstract]
Pension and Other Postretirement Benefits Cost (Reversal of Cost)			15	24
Gain (loss) on equity method investments			0	0
Provision for uncollectible accounts			20	(4)
Stock-based compensation costs			0	0
Other Decommissioning Related Activity			0	0
Energy-related options			0	0
Amortization of regulatory asset related to debt costs			1	1
Amortization of rate stabilization deferral			7	(21)
Amortization of debt fair value adjustment			0	(2)
Discrete impacts from EIMA			0	0
Amortization of debt costs			1	0
Inventory Write-down			0	0
Other			9	(6)
Total other noncash operating activities			53	(8)
Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract]
Capital Expenditures Incurred but Not yet Paid			61	(5)
Capitalized Costs, Asset Retirement Costs			0
Noncash or Part Noncash Acquisition, Fixed Assets Acquired				0
Stock Issued			$ 0	$ 0

[1] Included in Depreciation and amortization on the Registrants' Consolidated Statements of Operations and Comprehensive Income.

[2] Included in Operating revenues or Purchased power and fuel expense on the Registrants’ Consolidated Statements of Operations and Comprehensive Income.

[3] Included in Purchased power and fuel expense on the Registrants’ Consolidated Statements of Operations and Comprehensive Income.

[4] Included in Operating and maintenance expense on the Registrants’ Consolidated Statements of Operations and Comprehensive Income.

[5] Includes the elimination of NDT fund activity for the Regulatory Agreement Units, including the elimination of operating revenues, ARO accretion, ARC amortization, investment income and income taxes related to all NDT fund activity for these units. See Note 15 — Asset Retirement Obligations of the Exelon 2017 Form 10-K for additional information regarding the accounting for nuclear decommissioning.

[6] Includes option premiums reclassified to realized at the settlement of the underlying contracts and recorded in Operating revenues.

[7] Reflects the change in distribution rates pursuant to EIMA and FEJA, which allows for the recovery of distribution costs by a utility through a pre-established performance-based formula rate tariff. Beginning June 1, 2017, also reflects the change in energy efficiency rates pursuant to FEJA, which allows for the recovery of energy efficiency costs by a utility through a pre-established performance-based formula rate tariff. See Note 6 — Regulatory Matters for more information.