Income Taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Mar. 31, 2015	Dec. 31, 1999
Income Taxes [Line Items]
Tax positions for which there is uncertainty about the timing of tax benefits		$ 540	$ 1,129
Unrecognized Tax Benefits		116	1	$ 6
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		760
Potential interest expense from a successful IRS challenge of the like-kind exchange position		280
Taxes accrued		293	305		$ 65
Exelon Generation Co L L C [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits		112	2	5
Taxes accrued		186	248
Allocation of federal tax benefit under tax sharing agreement	77		55
Baltimore Gas and Electric Company [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits		0	0	0
Taxes accrued		23	42
Commonwealth Edison Co [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits		0	0	0
Deferred gain on sale of property					$ 35
Expected non-cash charge to earnings	172
Noncash contributions from parent	$ 172
Taxes accrued		63	59
Intercompany Non Cash Allocation Of Tax Benefit		202	273	0
PECO Energy Co [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits		0	0	$ 0
Taxes accrued		4	3
Allocation of federal tax benefit under tax sharing agreement			$ 25
Maximum [Member]
Income Taxes [Line Items]
Cash tax benefit (detriment) as a result of repair costs deduction		$ 120