Supplemental Financial Information - Supplemental Balance Sheet Information (Details) - USD ($)		Dec. 31, 2017	Dec. 31, 2016
Supplemental Balance Sheet Information [Line Items]
Total equity method investments		$ 297,000,000	$ 311,000,000
Investments		640,000,000	629,000,000
Compensation-related accruals	[1]	978,000,000	1,199,000,000
Taxes accrued		373,000,000	723,000,000
Interest accrued		328,000,000	1,234,000,000
Severance accrued		58,000,000	44,000,000
Other accrued expenses		98,000,000	260,000,000
Accrued Liabilities		1,835,000,000	3,460,000,000
Total accrued expenses		1,835,000,000	3,460,000,000
Financing Trusts [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	[2]	14,000,000	22,000,000
Bloom Energy [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		206,000,000	216,000,000
Net Power [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		76,000,000	57,000,000
Other Equity Method Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		1,000,000	16,000,000
Employee Benefit Trusts and Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Employee benefit trusts and investments	[3]	244,000,000	232,000,000
Other Cost Method Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Other cost method investments		62,000,000	52,000,000
Other Available For Sale Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Other available for sale investments		37,000,000	34,000,000
Exelon Generation Co L L C [Member]
Supplemental Balance Sheet Information [Line Items]
Total equity method investments		283,000,000	288,000,000
Investments		433,000,000	418,000,000
Compensation-related accruals	[1]	407,000,000	557,000,000
Taxes accrued		444,000,000	239,000,000
Interest accrued		78,000,000	82,000,000
Severance accrued		30,000,000	15,000,000
Other accrued expenses		61,000,000	96,000,000
Total accrued expenses		1,020,000,000	989,000,000
Exelon Generation Co L L C [Member] | Financing Trusts [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	[2]	0	0
Exelon Generation Co L L C [Member] | Bloom Energy [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		206,000,000	216,000,000
Exelon Generation Co L L C [Member] | Net Power [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		76,000,000	57,000,000
Exelon Generation Co L L C [Member] | Other Equity Method Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		1,000,000	15,000,000
Exelon Generation Co L L C [Member] | Employee Benefit Trusts and Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Employee benefit trusts and investments	[3]	51,000,000	44,000,000
Exelon Generation Co L L C [Member] | Other Cost Method Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Other cost method investments		62,000,000	52,000,000
Exelon Generation Co L L C [Member] | Other Available For Sale Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Other available for sale investments		37,000,000	34,000,000
Commonwealth Edison Co [Member]
Supplemental Balance Sheet Information [Line Items]
Total equity method investments		6,000,000	6,000,000
Investments		6,000,000	6,000,000
Compensation-related accruals	[1]	158,000,000	199,000,000
Taxes accrued		60,000,000	330,000,000
Interest accrued		102,000,000	609,000,000
Severance accrued		2,000,000	2,000,000
Other accrued expenses		5,000,000	110,000,000
Total accrued expenses		327,000,000	1,250,000,000
Commonwealth Edison Co [Member] | Financing Trusts [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	[2]	6,000,000	6,000,000
Commonwealth Edison Co [Member] | Bloom Energy [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		0	0
Commonwealth Edison Co [Member] | Net Power [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		0	0
Commonwealth Edison Co [Member] | Other Equity Method Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		0	0
Commonwealth Edison Co [Member] | Employee Benefit Trusts and Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Employee benefit trusts and investments	[3]	0	0
Commonwealth Edison Co [Member] | Other Cost Method Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Other cost method investments		0	0
Commonwealth Edison Co [Member] | Other Available For Sale Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Other available for sale investments		0	0
PECO Energy Co [Member]
Supplemental Balance Sheet Information [Line Items]
Total equity method investments		8,000,000	8,000,000
Investments		25,000,000	25,000,000
Compensation-related accruals	[1]	64,000,000	67,000,000
Taxes accrued		15,000,000	4,000,000
Interest accrued		33,000,000	30,000,000
Severance accrued		0	0
Other accrued expenses		2,000,000	3,000,000
Total accrued expenses		114,000,000	104,000,000
PECO Energy Co [Member] | Financing Trusts [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	[2]	8,000,000	8,000,000
PECO Energy Co [Member] | Bloom Energy [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		0	0
PECO Energy Co [Member] | Net Power [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		0	0
PECO Energy Co [Member] | Other Equity Method Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		0	0
PECO Energy Co [Member] | Employee Benefit Trusts and Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Employee benefit trusts and investments	[3]	17,000,000	17,000,000
PECO Energy Co [Member] | Other Cost Method Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Other cost method investments		0	0
PECO Energy Co [Member] | Other Available For Sale Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Other available for sale investments		0	0
Baltimore Gas and Electric Company [Member]
Supplemental Balance Sheet Information [Line Items]
Total equity method investments		0	8,000,000
Investments		5,000,000	12,000,000
Compensation-related accruals	[1]	58,000,000	64,000,000
Taxes accrued		71,000,000	78,000,000
Interest accrued		34,000,000	31,000,000
Severance accrued		0	0
Other accrued expenses		1,000,000	2,000,000
Total accrued expenses		164,000,000	175,000,000
Baltimore Gas and Electric Company [Member] | Financing Trusts [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	[2]	0	8,000,000
Baltimore Gas and Electric Company [Member] | Bloom Energy [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		0	0
Baltimore Gas and Electric Company [Member] | Net Power [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		0	0
Baltimore Gas and Electric Company [Member] | Other Equity Method Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		0	0
Baltimore Gas and Electric Company [Member] | Employee Benefit Trusts and Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Employee benefit trusts and investments	[3]	5,000,000	4,000,000
Baltimore Gas and Electric Company [Member] | Other Cost Method Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Other cost method investments		0	0
Baltimore Gas and Electric Company [Member] | Other Available For Sale Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Other available for sale investments		0	0
Pepco Holdings LLC [Member] | Successor [Member]
Supplemental Balance Sheet Information [Line Items]
Total equity method investments		0	0
Investments		132,000,000	133,000,000
Compensation-related accruals	[1]	106,000,000	112,000,000
Taxes accrued		61,000,000	65,000,000
Interest accrued		48,000,000	49,000,000
Severance accrued		17,000,000	19,000,000
Other accrued expenses		29,000,000	27,000,000
Total accrued expenses		261,000,000	272,000,000
Pepco Holdings LLC [Member] | Financing Trusts [Member] | Successor [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	[2]	0	0
Pepco Holdings LLC [Member] | Bloom Energy [Member] | Successor [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		0	0
Pepco Holdings LLC [Member] | Net Power [Member] | Successor [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		0	0
Pepco Holdings LLC [Member] | Other Equity Method Investments [Member] | Successor [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		0	0
Pepco Holdings LLC [Member] | Employee Benefit Trusts and Investments [Member] | Successor [Member]
Supplemental Balance Sheet Information [Line Items]
Employee benefit trusts and investments		132,000,000	133,000,000
Pepco Holdings LLC [Member] | Other Cost Method Investments [Member] | Successor [Member]
Supplemental Balance Sheet Information [Line Items]
Other cost method investments		0	0
Pepco Holdings LLC [Member] | Other Available For Sale Investments [Member] | Successor [Member]
Supplemental Balance Sheet Information [Line Items]
Other available for sale investments		0	0
Potomac Electric Power Company [Member]
Supplemental Balance Sheet Information [Line Items]
Total equity method investments		0	0
Investments		102,000,000	102,000,000
Compensation-related accruals	[1]	29,000,000	30,000,000
Taxes accrued		68,000,000	48,000,000
Interest accrued		23,000,000	21,000,000
Severance accrued		0	0
Other accrued expenses		17,000,000	14,000,000
Total accrued expenses		137,000,000	113,000,000
Potomac Electric Power Company [Member] | Financing Trusts [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	[2]	0	0
Potomac Electric Power Company [Member] | Bloom Energy [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		0	0
Potomac Electric Power Company [Member] | Net Power [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		0	0
Potomac Electric Power Company [Member] | Other Equity Method Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		0	0
Potomac Electric Power Company [Member] | Employee Benefit Trusts and Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Employee benefit trusts and investments		102,000,000	102,000,000
Potomac Electric Power Company [Member] | Other Cost Method Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Other cost method investments		0	0
Potomac Electric Power Company [Member] | Other Available For Sale Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Other available for sale investments		0	0
Delmarva Power and Light Company [Member]
Supplemental Balance Sheet Information [Line Items]
Total equity method investments		0	0
Investments		0	0
Compensation-related accruals	[1]	17,000,000	17,000,000
Taxes accrued		4,000,000	4,000,000
Interest accrued		8,000,000	8,000,000
Severance accrued		0	0
Other accrued expenses		6,000,000	7,000,000
Total accrued expenses		35,000,000	36,000,000
Delmarva Power and Light Company [Member] | Financing Trusts [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	[2]	0	0
Delmarva Power and Light Company [Member] | Bloom Energy [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		0	0
Delmarva Power and Light Company [Member] | Net Power [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		0	0
Delmarva Power and Light Company [Member] | Other Equity Method Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		0	0
Delmarva Power and Light Company [Member] | Employee Benefit Trusts and Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Employee benefit trusts and investments		0	0
Delmarva Power and Light Company [Member] | Other Cost Method Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Other cost method investments		0	0
Delmarva Power and Light Company [Member] | Other Available For Sale Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Other available for sale investments		0	0
Atlantic City Electric Company [Member]
Supplemental Balance Sheet Information [Line Items]
Total equity method investments		0	0
Investments		0	0
Compensation-related accruals	[1]	11,000,000	11,000,000
Taxes accrued		5,000,000	9,000,000
Interest accrued		12,000,000	12,000,000
Severance accrued		0	0
Other accrued expenses		5,000,000	6,000,000
Total accrued expenses		33,000,000	38,000,000
Atlantic City Electric Company [Member] | Financing Trusts [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments	[2]	0	0
Atlantic City Electric Company [Member] | Bloom Energy [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		0	0
Atlantic City Electric Company [Member] | Net Power [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		0	0
Atlantic City Electric Company [Member] | Other Equity Method Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Equity Method Investments		0	0
Atlantic City Electric Company [Member] | Employee Benefit Trusts and Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Employee benefit trusts and investments		0	0
Atlantic City Electric Company [Member] | Other Cost Method Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Other cost method investments		0	0
Atlantic City Electric Company [Member] | Other Available For Sale Investments [Member]
Supplemental Balance Sheet Information [Line Items]
Other available for sale investments		$ 0	$ 0

[1] Primarily includes accrued payroll, bonuses and other incentives, vacation and benefits.

[2] Includes investments in affiliated financing trusts, which were not consolidated within the financial statements of Exelon and are shown as investments on the Consolidated Balance Sheets. See Note 1 — Significant Accounting Policies for additional information.

[3] The Registrants’ investments in these marketable securities are recorded at fair market value.