Supplemental Financial Information - Balance Sheet (Details) - USD ($) $ in Millions	Mar. 31, 2018		Dec. 31, 2017		Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Property, Plant and Equipment, Net [Abstract]
Accumulated depreciation	$ 21,905	[1]	$ 21,064	[2]
Accounts receivable, net
Allowance for uncollectible accounts	369		322
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash, Cash Equivalents and Restricted Cash at End of Period	1,099		1,190		$ 889	$ 914	$ 889	$ 914
Cash and Cash Equivalents, at Carrying Value	787		898		609	635
Restricted Cash, Noncurrent	103		85		26	26
Restricted cash	209		207		254	253
Exelon Generation Co L L C [Member]
Property, Plant and Equipment, Net [Abstract]
Accumulated depreciation	11,936	[1]	11,428	[2]
Accounts receivable, net
Allowance for uncollectible accounts	115		114
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash, Cash Equivalents and Restricted Cash at End of Period	737		554		540	448
Cash and Cash Equivalents, at Carrying Value	610		416		400	290
Restricted Cash, Noncurrent	0		0		0	0
Restricted cash	127		138		140	158
Exelon Generation Co L L C [Member] | Nuclear Fuel [Member]
Property, Plant and Equipment, Net [Abstract]
Accumulated depreciation	3,263		3,159
Commonwealth Edison Co [Member]
Property, Plant and Equipment, Net [Abstract]
Accumulated depreciation	4,391		4,269
Accounts receivable, net
Allowance for uncollectible accounts	89		73
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash, Cash Equivalents and Restricted Cash at End of Period	162		144		34	58
Cash and Cash Equivalents, at Carrying Value	70		76		31	56
Restricted Cash, Noncurrent	83		63		0	0
Restricted cash	9		5		3	2
PECO Energy Co [Member]
Property, Plant and Equipment, Net [Abstract]
Accumulated depreciation	3,445		3,411
Accounts receivable, net
Allowance for uncollectible accounts	68		56
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash, Cash Equivalents and Restricted Cash at End of Period	26		275		32	67
Cash and Cash Equivalents, at Carrying Value	21		271		28	63
Restricted Cash, Noncurrent	0		0		0	0
Restricted cash	5		4		4	4
Baltimore Gas and Electric Company [Member]
Property, Plant and Equipment, Net [Abstract]
Accumulated depreciation	3,471		3,405
Accounts receivable, net
Allowance for uncollectible accounts	31		24
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash, Cash Equivalents and Restricted Cash at End of Period	24		18		57	50
Cash and Cash Equivalents, at Carrying Value	22		17		11	23
Restricted Cash, Noncurrent	0		0		3	3
Restricted cash	2		1		43	24
Pepco Holdings LLC [Member]
Property, Plant and Equipment, Net [Abstract]
Accumulated depreciation	575		487
Accounts receivable, net
Allowance for uncollectible accounts	66		55
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash, Cash Equivalents and Restricted Cash at End of Period	103		95		173	236
Cash and Cash Equivalents, at Carrying Value	43		30		109	170
Restricted Cash, Noncurrent	20		23		23	23
Restricted cash	40		42		41	43
Potomac Electric Power Company [Member]
Property, Plant and Equipment, Net [Abstract]
Accumulated depreciation	3,224		3,177
Accounts receivable, net
Allowance for uncollectible accounts	24		21
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash, Cash Equivalents and Restricted Cash at End of Period	48		40		41	42
Cash and Cash Equivalents, at Carrying Value	15		5		8	9
Restricted Cash, Noncurrent	0		0		0	0
Restricted cash	33		35		33	33
Delmarva Power and Light Company [Member]
Property, Plant and Equipment, Net [Abstract]
Accumulated depreciation	1,273		1,247
Accounts receivable, net
Allowance for uncollectible accounts	22		16
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash, Cash Equivalents and Restricted Cash at End of Period	7		2		44	46
Cash and Cash Equivalents, at Carrying Value	7		2		44	46
Restricted Cash, Noncurrent	0		0		0	0
Restricted cash	0		0		0	0
Atlantic City Electric Company [Member]
Property, Plant and Equipment, Net [Abstract]
Accumulated depreciation	1,086		1,066
Accounts receivable, net
Allowance for uncollectible accounts	20		18
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash, Cash Equivalents and Restricted Cash at End of Period	37		31		84	133
Cash and Cash Equivalents, at Carrying Value	10		2		54	101
Restricted Cash, Noncurrent	20		23		23	23
Restricted cash	$ 7		$ 6		$ 7	$ 9

[1] Includes accumulated amortization of nuclear fuel in the reactor core of $3,263 million.

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