Income Taxes - Narrative (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended			12 Months Ended				3 Months Ended
	Sep. 30, 2012	Dec. 31, 2014	Mar. 31, 2014	Mar. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Mar. 31, 2013	Dec. 31, 1999
Income Taxes [Line Items]
Tax positions for which there is uncertainty about the timing of tax benefits		$ 1,129			$ 1,129	$ 1,387
Unrecognized tax benefits that if recognized would affect the effective tax rate		701			701	788
Unrecognized Tax Benefits					1	6	6
Increase in unrecognized tax benefits resulting from settlements with taxing authorities					188
Deferred gain on sale of property										1,200
IRS asserted penalties for understatement of tax					90
Expected non-cash charge to earnings									265
Potential tax and interest from a successful IRS challenge of the like-kind exchange transaction position		810			810
Potential interest expense from a successful IRS challenge of the like-kind exchange position		310			310
Proceeds from long-term capital lease obligations			335
Taxes accrued		305	285		305	315
Cash tax benefit (detriment) as a result of repair costs deduction								300
Effective Income Tax Rate Reconciliation, Deduction, Other, Amount								35
Gas Distribution Repair Tax Benefit Expense	26
Deferred State Tax Asset From State Tax Apportionment				72
Income tax benefit recorded as a result of re-apportionment of state income taxes		3		116
Deferred state tax liability resulting from purchase accounting				44
Exelon Generation Co L L C [Member]
Income Taxes [Line Items]
Unrecognized tax benefits that if recognized would affect the effective tax rate		672			672	768
Unrecognized Tax Benefits		661			2	5	6
Taxes accrued		248			248	212
Cash tax benefit (detriment) as a result of repair costs deduction								28
Income tax benefit recorded as a result of re-apportionment of state income taxes		7		14
Deferred state tax liability resulting from purchase accounting				14
Allocation of federal tax benefit under tax sharing agreement			77			26
Allocation of tax benefit from parent							48
Baltimore Gas and Electric Company [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits					0	0	0
Taxes accrued		42			42	16
Cash tax benefit (detriment) as a result of repair costs deduction							27
Commonwealth Edison Co [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits					0	0	0
Deferred gain on sale of property			155
Expected non-cash charge to earnings									170
Noncash contributions from parent									172
Taxes accrued		59			59	62
Cash tax benefit (detriment) as a result of repair costs deduction								250
Intercompany Non Cash Allocation Of Tax Benefit					273	0	11
PECO Energy Co [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits					0	0	0
Taxes accrued		3			3	24
Cash tax benefit (detriment) as a result of repair costs deduction								95
Gas Distribution Repair Tax Benefit Expense	29
Allocation of federal tax benefit under tax sharing agreement						27
Allocation of tax benefit from parent						27	9
Maximum [Member]
Income Taxes [Line Items]
Cash tax benefit (detriment) as a result of repair costs deduction					$ 120