Supplemental Financial Information - Cash Flow (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Depreciation, Amortization and Accretion [Abstract]
Property, plant and equipment	$ 471		$ 354
Regulatory assets	61		25
Amortization of intangible assets, net	11		3
Amortization of energy contract assets and liabilities	176	[1]	131	[1]
Nuclear fuel	230	[1]	207	[1]
Asset retirement obligation accretion	68	[2]	56	[2]
Total depreciation, amortization and accretion	1,017		776
Other Non-Cash Operating Activities [Abstract]
Pension and non-pension postretirement benefits costs	205		181
Provision for uncollectible accounts	45		40
Stock-based compensation costs	39		39
Other Decommissioning Related Activity	(64)		(71)	[3]
Energy-related options	21		28	[4]
Amortization of regulatory asset related to debt costs	4		6
Amortization of rate stabilization deferral	30		3
Amortization of debt fair value adjustment	(9)
Uncollectible accounts recovery			0
Discrete impacts from 2010 Rate Case Order			(38)	[5]
Bargain purchase gain			0
Discrete impacts from EIMA	(49)	[5]
Merger related commitments	(6)	[5]	331	[6]
Gain (loss) on equity method investments	9		22
Amortization of debt costs	5
Other	1		(11)
Total other noncash operating activities	231		530
Changes In Other Assets and Liabilities [Abstract]
Under/over-recovered energy and transmission costs	29		(25)
Other regulatory assets and liabilities	(91)		97
Other current assets and liabilities	(169)		(18)
Other noncurrent assets and liabilities	282		18
Total changes in other assets and liabilities	233		(122)
Cash Flow Noncash Investing And Financing Activities Disclosure [Abstract]
Merger with Constellation, common stock issued			7,365
Consolidated VIE dividend to non-controlling interest	(63)
Total noncash investing and financing activities	63		7,365
DOE Smart Grid Investment Grant [Abstract]
Amount included in capital expenditures	21		26
Smart Grid Grant Reimbursements	32		35
Exelon Generation Co L L C [Member]
Depreciation, Amortization and Accretion [Abstract]
Property, plant and equipment	203		150
Amortization of intangible assets, net	11		3
Amortization of energy contract assets and liabilities	176	[1]	131	[1]
Nuclear fuel	230	[1]	207	[1]
Asset retirement obligation accretion	68	[2]	56	[2]
Total depreciation, amortization and accretion	688		547
Other Non-Cash Operating Activities [Abstract]
Pension and non-pension postretirement benefits costs	87		81
Provision for uncollectible accounts	7		(9)
Stock-based compensation costs	4
Other Decommissioning Related Activity	(64)		(71)	[3]
Energy-related options	21		28	[4]
Amortization of debt fair value adjustment	(9)
Bargain purchase gain			0
Merger related commitments			35	[6]
Gain (loss) on equity method investments	9		22
Amortization of debt costs	3
Other	8		4
Total other noncash operating activities	66		90
Changes In Other Assets and Liabilities [Abstract]
Other current assets and liabilities	(131)		(122)
Other noncurrent assets and liabilities	(28)		42
Total changes in other assets and liabilities	(159)		(80)
Cash Flow Noncash Investing And Financing Activities Disclosure [Abstract]
Merger with Constellation, common stock issued			5,272
Consolidated VIE dividend to non-controlling interest	(63)
Total noncash investing and financing activities	63		5,272
Commonwealth Edison Co [Member]
Depreciation, Amortization and Accretion [Abstract]
Property, plant and equipment	137		131
Regulatory assets	30		18
Total depreciation, amortization and accretion	167		149
Other Non-Cash Operating Activities [Abstract]
Pension and non-pension postretirement benefits costs	77		69
Provision for uncollectible accounts	9		22
Stock-based compensation costs	1
Amortization of regulatory asset related to debt costs	3		4
Uncollectible accounts recovery			0
Discrete impacts from 2010 Rate Case Order			(38)	[5]
Discrete impacts from EIMA	(49)	[5]
Amortization of debt costs	1
Other	0		3
Total other noncash operating activities	42		60
Changes In Other Assets and Liabilities [Abstract]
Under/over-recovered energy and transmission costs	(18)		(38)
Other regulatory assets and liabilities	14		16
Other current assets and liabilities	0		1
Other noncurrent assets and liabilities	263		(19)
Total changes in other assets and liabilities	231		(72)
Cash Flow Noncash Investing And Financing Activities Disclosure [Abstract]
Indemnification of like-kind exchange position	0		0
Total noncash investing and financing activities	172
Commonwealth Edison Co [Member] | Indemnification Agreement [Member]
Cash Flow Noncash Investing And Financing Activities Disclosure [Abstract]
Indemnification of like-kind exchange position	172	[7]
PECO Energy Co [Member]
Depreciation, Amortization and Accretion [Abstract]
Property, plant and equipment	55		51
Regulatory assets	2		2
Total depreciation, amortization and accretion	57		53
Other Non-Cash Operating Activities [Abstract]
Pension and non-pension postretirement benefits costs	11		13
Provision for uncollectible accounts	25		24
Stock-based compensation costs	1
Amortization of regulatory asset related to debt costs	1		1
Amortization of debt costs	1
Other	0		2
Total other noncash operating activities	39		40
Changes In Other Assets and Liabilities [Abstract]
Under/over-recovered energy and transmission costs	22		13
Other regulatory assets and liabilities	(13)		(3)
Other current assets and liabilities	(75)		(134)	[8]
Other noncurrent assets and liabilities	2		8
Total changes in other assets and liabilities	(38)		(110)
Cash Flow Noncash Investing And Financing Activities Disclosure [Abstract]
Indemnification of like-kind exchange position	0		0
DOE Smart Grid Investment Grant [Abstract]
Amount included in capital expenditures	6		20
Smart Grid Grant Reimbursements	12		21
Baltimore Gas and Electric Company [Member]
Depreciation, Amortization and Accretion [Abstract]
Property, plant and equipment	64		61
Regulatory assets	29		18
Total depreciation, amortization and accretion	93		79
Other Non-Cash Operating Activities [Abstract]
Pension and non-pension postretirement benefits costs	14		16
Provision for uncollectible accounts	4		8
Stock-based compensation costs	1
Amortization of regulatory asset related to debt costs	0		1
Amortization of rate stabilization deferral	30		13
Deferral of storm costs			0
Merger related commitments	(6)	[5]	141	[6]
Other	(1)		0
Total other noncash operating activities	42		179
Changes In Other Assets and Liabilities [Abstract]
Under/over-recovered energy and transmission costs	16		(10)
Other regulatory assets and liabilities	53		39
Other current assets and liabilities	73		31
Other noncurrent assets and liabilities	(2)		7
Total changes in other assets and liabilities	34		(11)
Cash Flow Noncash Investing And Financing Activities Disclosure [Abstract]
Indemnification of like-kind exchange position	0
DOE Smart Grid Investment Grant [Abstract]
Amount included in capital expenditures	15		6
Smart Grid Grant Reimbursements	$ 20		$ 14

[1] Included in revenues or fuel expense on the Registrants' Consolidated Statements of Operations and Comprehensive Income.

[2] Included in operating and maintenance expense on the Registrants' Consolidated Statements of Operations.

[3] Includes the elimination of NDT fund-related activity for the Regulatory Agreement Units, including the elimination of operating revenues, ARO accretion, ARC amortization, investment income and income taxes related to all NDT fund activity for these units. See Note 13 of the Exelon 2012 Form 10-K for additional information regarding the accounting for nuclear decommissioning.

[4] Includes option premiums reclassified to realized at the settlement of the underlying contracts and recorded to results of operations.

[5] Reflects the change in distribution rates pursuant to EIMA, which allows for the recovery of costs by a utility through a pre-established performance-based formula rate tariff. See Note 5 - Regulatory Matters for more information.

[6] See Note 4 - Mergers and Acquisitions for more information on merger-related commitments.

[7] See Note 11 – Income Taxes for discussion of the like-kind exchange tax position.

[8] Relates primarily to prepaid utility taxes.