Supplemental Financial Information - Cash Flow (Details) (USD $)	3 Months Ended	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2012		Sep. 30, 2011
Depreciation, Amortization and Accretion [Abstract]
Property, plant and equipment		$ 1,263,000,000		$ 947,000,000
Regulatory assets		89,000,000		40,000,000
Amortization of intangible assets		24,000,000
Amortization of energy contract assets and liabilities		731,000,000	[1]
Nuclear fuel		628,000,000	[1]	556,000,000	[1]
Asset retirement obligation accretion		174,000,000	[2]	159,000,000	[2]
Total depreciation, amortization and accretion		2,909,000,000		1,702,000,000
Other Non-Cash Operating Activities [Abstract]
Pension and non-pension postretirement benefits costs		606,000,000		407,000,000
Provision for uncollectible accounts		120,000,000		97,000,000
Stock-based compensation costs		75,000,000		55,000,000
Other Decommissioning Related Activity		(108,000,000)	[3]	62,000,000	[3]
Energy-related options		119,000,000	[4]	102,000,000	[4]
Amortization of regulatory asset related to debt costs		13,000,000		16,000,000
Amortization of rate stabilization deferral		39,000,000
Amortization of debt fair value adjustment		(49,000,000)
Uncollectible accounts recovery				14,000,000
Discrete impacts from 2010 Rate Case Order				(32,000,000)	[5]
Bargain purchase gain	(36,000,000)			(36,000,000)
Discrete impacts from EIMA		43,000,000	[6]
Merger related commitments		179,000,000	[7]
Severance Costs		120,000,000
Gain (loss) on equity method investments		69,000,000
Other		9,000,000		18,000,000
Total other noncash operating activities		1,235,000,000		703,000,000
Changes In Other Assets and Liabilities [Abstract]
Under/over-recovered energy and transmission costs		20,000,000		(9,000,000)
Other regulatory assets and liabilities		463,000,000		0
Other current assets and liabilities		52,000,000		(166,000,000)
Other noncurrent assets and liabilities		(40,000,000)		83,000,000
Total changes in other assets and liabilities		(431,000,000)		(92,000,000)
Cash Flow Noncash Investing And Financing Activities Disclosure [Abstract]
Merger with Constellation, common stock issued		7,365,000,000
Total noncash investing and financing activities		7,365,000,000
DOE Smart Grid Investment Grant [Abstract]
Amount included in capital expenditures		75,000,000		47,000,000
Smart Grid Grant Reimbursements		85,000,000		86,000,000
Exelon Generation Co L L C [Member]
Depreciation, Amortization and Accretion [Abstract]
Property, plant and equipment		540,000,000		416,000,000
Amortization of intangible assets		24,000,000
Amortization of energy contract assets and liabilities		811,000,000	[1]
Nuclear fuel		629,000,000	[1]	556,000,000	[1]
Asset retirement obligation accretion		174,000,000	[2]	159,000,000	[2]
Total depreciation, amortization and accretion		2,178,000,000		1,131,000,000
Other Non-Cash Operating Activities [Abstract]
Pension and non-pension postretirement benefits costs		259,000,000		187,000,000
Provision for uncollectible accounts		14,000,000		0
Other Decommissioning Related Activity		(108,000,000)	[3]	62,000,000	[3]
Energy-related options		119,000,000	[4]	102,000,000	[4]
Amortization of debt fair value adjustment		(23,000,000)
Bargain purchase gain	(36,000,000)			(36,000,000)
Merger related commitments		35,000,000	[7]
Severance Costs		34,000,000
Gain (loss) on equity method investments		69,000,000
Other		23,000,000		47,000,000
Total other noncash operating activities		422,000,000		362,000,000
Changes In Other Assets and Liabilities [Abstract]
Other current assets and liabilities		(85,000,000)		(46,000,000)
Other noncurrent assets and liabilities		(110,000,000)		(19,000,000)
Total changes in other assets and liabilities		(195,000,000)		(65,000,000)
Cash Flow Noncash Investing And Financing Activities Disclosure [Abstract]
Merger with Constellation, common stock issued		5,258,000,000
Total noncash investing and financing activities		5,258,000,000
Commonwealth Edison Co [Member]
Depreciation, Amortization and Accretion [Abstract]
Property, plant and equipment		396,000,000		374,000,000
Regulatory assets		62,000,000		31,000,000
Total depreciation, amortization and accretion		458,000,000		405,000,000
Other Non-Cash Operating Activities [Abstract]
Pension and non-pension postretirement benefits costs		212,000,000		160,000,000
Provision for uncollectible accounts		38,000,000		49,000,000
Amortization of regulatory asset related to debt costs		10,000,000		14,000,000
Uncollectible accounts recovery				14,000,000
Discrete impacts from 2010 Rate Case Order				(32,000,000)	[5]
Discrete impacts from EIMA		43,000,000	[6]
Other		7,000,000		5,000,000
Total other noncash operating activities		310,000,000		210,000,000
Changes In Other Assets and Liabilities [Abstract]
Under/over-recovered energy and transmission costs		21,000,000		(20,000,000)
Other regulatory assets and liabilities		65,000,000		(12,000,000)
Other current assets and liabilities		(8,000,000)		(13,000,000)
Other noncurrent assets and liabilities		(72,000,000)		50,000,000
Total changes in other assets and liabilities		(124,000,000)		29,000,000
PECO Energy Co [Member]
Depreciation, Amortization and Accretion [Abstract]
Property, plant and equipment		154,000,000		141,000,000
Regulatory assets		7,000,000		9,000,000
Total depreciation, amortization and accretion		161,000,000		150,000,000
Other Non-Cash Operating Activities [Abstract]
Pension and non-pension postretirement benefits costs		38,000,000		24,000,000
Provision for uncollectible accounts		46,000,000		48,000,000
Amortization of regulatory asset related to debt costs		2,000,000		2,000,000
Severance Costs		1,000,000
Other		9,000,000		0
Total other noncash operating activities		96,000,000		74,000,000
Changes In Other Assets and Liabilities [Abstract]
Under/over-recovered energy and transmission costs		(3,000,000)		11,000,000
Other regulatory assets and liabilities		(7,000,000)		(13,000,000)
Other current assets and liabilities		(56,000,000)	[8]	(59,000,000)
Other noncurrent assets and liabilities		(5,000,000)		7,000,000
Total changes in other assets and liabilities		(57,000,000)		(28,000,000)
DOE Smart Grid Investment Grant [Abstract]
Amount included in capital expenditures		45,000,000		29,000,000
Smart Grid Grant Reimbursements		55,000,000		45,000,000
Baltimore Gas and Electric Company [Member]
Depreciation, Amortization and Accretion [Abstract]
Property, plant and equipment		184,000,000		170,000,000
Regulatory assets		34,000,000		35,000,000
Total depreciation, amortization and accretion		218,000,000		205,000,000
Other Non-Cash Operating Activities [Abstract]
Pension and non-pension postretirement benefits costs		44,000,000		43,000,000
Provision for uncollectible accounts		28,000,000		31,000,000
Amortization of regulatory asset related to debt costs		1,000,000		1,000,000
Amortization of rate stabilization deferral		49,000,000		45,000,000
Deferral of storm costs				(16,000,000)
Merger related commitments		28,000,000	[7]
Other		(3,000,000)		(6,000,000)
Total other noncash operating activities		147,000,000		98,000,000
Changes In Other Assets and Liabilities [Abstract]
Under/over-recovered energy and transmission costs		21,000,000		(25,000,000)
Other regulatory assets and liabilities		89,000,000		(13,000,000)
Other current assets and liabilities		(25,000,000)		(92,000,000)
Other noncurrent assets and liabilities		7,000,000		(50,000,000)
Total changes in other assets and liabilities		(86,000,000)		(154,000,000)
DOE Smart Grid Investment Grant [Abstract]
Amount included in capital expenditures		30,000,000		18,000,000
Smart Grid Grant Reimbursements		$ 30,000,000		$ 41,000,000

[1] Included in purchased power and 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 and Comprehensive Income

[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 12 of the Exelon 2011 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] In May 2011, as a result of the 2010 Rate Case order, ComEd recorded one-time net benefits to reestablish previously expensed plant balances and to recover previously incurred costs related to Exelon's 2009 restructuring plan. See Note 4 - Regulatory Matters for more information.

[6] Includes the establishment of a regulatory asset, pursuant to EIMA, which represents the ICC’s approved distribution formula and associated rulings as of September 30, 2012 and ComEd’s best estimate of the probable increase in distribution rates to provide recovery of prudent and reasonable costs incurred for the twelve months ended December 31, 2011 and the nine months ended September 30, 2012. See Note 4 - Regulatory Matters for more information.

[7] See Note 3 - Mergers and Acquisitions for more information on merger-related commitments.

[8] Relates primarily to prepaid utility taxes.