Retirement Benefits - Allocated Portion of Pension and Postretirement Benefit Plan Costs (Details) - USD ($) $ in Millions		3 Months Ended						6 Months Ended
		Jun. 30, 2017		Jun. 30, 2016		Jun. 30, 2016	Mar. 23, 2016	Jun. 30, 2017		Jun. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Cost (Reversal of Cost)		$ 163		$ 160				$ 320		$ 297
Corporate, Non-Segment [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Cost (Reversal of Cost)	[1]	13		10				26		24
Exelon Generation Co L L C [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Cost (Reversal of Cost)		59	[2]	55				113	[2]	109
Commonwealth Edison Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Cost (Reversal of Cost)		44		42				87		83
PECO Energy Co [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Cost (Reversal of Cost)		7		8				14		17
Baltimore Gas and Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Cost (Reversal of Cost)		16		18				31		33
Baltimore Gas and Electric Company [Member] | Other Pension, Postretirement and Supplemental Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Cost (Reversal of Cost)								32
Potomac Electric Power Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Cost (Reversal of Cost)		6	[3]	7	[3]		$ 7	13	[3]	16	[3]
Delmarva Power and Light Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Cost (Reversal of Cost)		3	[3]	4	[3]		4	6	[3]	9	[3]
Atlantic City Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Cost (Reversal of Cost)		3	[3]	4	[3]		3	7	[3]	8	[3]
Pepco Holdings LLC [Member] | Successor [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Cost (Reversal of Cost)		24				$ 31		48
Pepco Holdings LLC [Member] | Predecessor [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Cost (Reversal of Cost)				27		$ 31	23
Pepco Holdings LLC [Member] | Corporate, Non-Segment [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefits Cost (Reversal of Cost)		$ 12	[1],[3]	$ 12	[1],[3]		$ 9	$ 22	[1],[3]	$ 21	[1],[3]

[1] These amounts primarily represent amounts billed to Exelon’s subsidiaries through intercompany allocations. These amounts are not included in the Generation, ComEd, PECO, BGE, PHI, Pepco, DPL or ACE amounts above.

[2] (c)FitzPatrick net benefit costs are included for the period after acquisition.

[3] Pepco's, DPL's, ACE's and PHISCO's pension and postretirement benefit costs for the six months ended June 30, 2016 include $7 million, $4 million, $3 million and $9 million, respectively, of costs incurred prior to the closing of Exelon’s merger with PHI on March 23, 2016.