Supplemental Financial Information - Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2018		Dec. 31, 2017		Sep. 30, 2017	Dec. 31, 2016
Property, Plant and Equipment, Net [Abstract]
Accumulated depreciation	$ 23,122	[1]	$ 21,064	[2]
Accounts receivable, net
Allowance for uncollectible accounts	354		322
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and Cash Equivalents, at Carrying Value	1,918		898		$ 1,203	$ 635
Restricted cash	240		207		320	253
Restricted Cash, Noncurrent	163		85		22	26
Cash, Cash Equivalents and Restricted Cash at End of Period	2,321		1,190		1,545	914
Nuclear Fuel [Member]
Property, Plant and Equipment, Net [Abstract]
Accumulated depreciation	3,278		3,159
Exelon Generation Co L L C [Member]
Property, Plant and Equipment, Net [Abstract]
Accumulated depreciation	12,753	[1]	11,428	[2]
Accounts receivable, net
Allowance for uncollectible accounts	112		114
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and Cash Equivalents, at Carrying Value	1,187		416		360	290
Restricted cash	152		138		186	158
Restricted Cash, Noncurrent	0		0		0	0
Cash, Cash Equivalents and Restricted Cash at End of Period	1,339		554		546	448
Exelon Generation Co L L C [Member] | Nuclear Fuel [Member]
Property, Plant and Equipment, Net [Abstract]
Accumulated depreciation	3,278		3,159
Commonwealth Edison Co [Member]
Property, Plant and Equipment, Net [Abstract]
Accumulated depreciation	4,557		4,269
Accounts receivable, net
Allowance for uncollectible accounts	93		73
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and Cash Equivalents, at Carrying Value	124		76		257	56
Restricted cash	12		5		52	2
Restricted Cash, Noncurrent	144		63		0	0
Cash, Cash Equivalents and Restricted Cash at End of Period	280		144		309	58
PECO Energy Co [Member]
Property, Plant and Equipment, Net [Abstract]
Accumulated depreciation	3,521		3,411
Accounts receivable, net
Allowance for uncollectible accounts	61		56
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and Cash Equivalents, at Carrying Value	102		271		330	63
Restricted cash	5		4		4	4
Restricted Cash, Noncurrent	0		0		0	0
Cash, Cash Equivalents and Restricted Cash at End of Period	107		275		334	67
Baltimore Gas and Electric Company [Member]
Property, Plant and Equipment, Net [Abstract]
Accumulated depreciation	3,579		3,405
Accounts receivable, net
Allowance for uncollectible accounts	22		24
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and Cash Equivalents, at Carrying Value	113		17		29	23
Restricted cash	3		1		1	24
Restricted Cash, Noncurrent	0		0		0	3
Cash, Cash Equivalents and Restricted Cash at End of Period	116		18		30	50
Pepco Holdings LLC [Member]
Property, Plant and Equipment, Net [Abstract]
Accumulated depreciation	759		487
Accounts receivable, net
Allowance for uncollectible accounts	66		55
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and Cash Equivalents, at Carrying Value	153		30		137	170
Restricted cash	42		42		43	43
Restricted Cash, Noncurrent	19		23		22	23
Cash, Cash Equivalents and Restricted Cash at End of Period	214		95		202	236
Potomac Electric Power Company [Member]
Property, Plant and Equipment, Net [Abstract]
Accumulated depreciation	3,311		3,177
Accounts receivable, net
Allowance for uncollectible accounts	25		21
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and Cash Equivalents, at Carrying Value	12		5		117	9
Restricted cash	35		35		34	33
Restricted Cash, Noncurrent	0		0		0	0
Cash, Cash Equivalents and Restricted Cash at End of Period	47		40		151	42
Delmarva Power and Light Company [Member]
Property, Plant and Equipment, Net [Abstract]
Accumulated depreciation	1,315		1,247
Accounts receivable, net
Allowance for uncollectible accounts	16		16
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and Cash Equivalents, at Carrying Value	110		2		3	46
Restricted cash	0		0		0	0
Restricted Cash, Noncurrent	0		0		0	0
Cash, Cash Equivalents and Restricted Cash at End of Period	110		2		3	46
Atlantic City Electric Company [Member]
Property, Plant and Equipment, Net [Abstract]
Accumulated depreciation	1,121		1,066
Accounts receivable, net
Allowance for uncollectible accounts	25		18
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and Cash Equivalents, at Carrying Value	11		2		5	101
Restricted cash	7		6		9	9
Restricted Cash, Noncurrent	19		23		22	23
Cash, Cash Equivalents and Restricted Cash at End of Period	$ 37		$ 31		$ 36	$ 133

[1] Includes accumulated amortization of nuclear fuel in the reactor core of $3,278 million.

[2] Includes accumulated amortization of nuclear fuel in the reactor core of $3,159 million.