Supplemental Financial Information - Cash Flow (Details) (USD $)	6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011
Depreciation, Amortization and Accretion [Abstract]
Property, plant and equipment	$ 806,000,000		$ 629,000,000
Regulatory assets	56,000,000		27,000,000
Amortization of intangible assets	14,000,000
Amortization of energy contract assets and liabilities	485,000,000	[1]
Nuclear fuel	419,000,000	[1]	355,000,000	[1]
Asset retirement obligation accretion	115,000,000	[2]	103,000,000	[2]
Total depreciation, amortization and accretion	1,895,000,000		1,114,000,000
Other Non-Cash Operating Activities [Abstract]
Pension and non-pension postretirement benefits costs	400,000,000		271,000,000
Provision for uncollectible accounts	60,000,000		45,000,000
Stock-based compensation costs	56,000,000		43,000,000
Other Decommissioning Related Activity	(62,000,000)	[3]	(35,000,000)	[3]
Energy-related options	64,000,000	[4]	68,000,000	[4]
Amortization of regulatory asset related to debt costs	9,000,000		11,000,000
Amortization of rate stabilization deferral	15,000,000
Amortization of debt fair value adjustment	(26,000,000)
Uncollectible accounts recovery			13,000,000
Discrete impacts from 2010 Rate Case Order			(32,000,000)	[5]
Discrete impacts from EIMA	69,000,000	[6]
Merger related commitments	188,000,000	[7]
Severance Costs	119,000,000
Gain (loss) on equity method investments	79,000,000
Other	(12,000,000)		(6,000,000)
Total other noncash operating activities	959,000,000		378,000,000
Changes In Other Assets and Liabilities [Abstract]
Under/over-recovered energy and transmission costs	(37,000,000)		(99,000,000)
Other regulatory assets and liabilities	514,000,000		0
Other current assets and liabilities	190,000,000		(218,000,000)
Other noncurrent assets and liabilities	4,000,000		68,000,000
Total changes in other assets and liabilities	(357,000,000)		(249,000,000)
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	68,000,000		34,000,000
Smart Grid Grant Reimbursements	65,000,000		54,000,000
Exelon Generation Co L L C [Member]
Depreciation, Amortization and Accretion [Abstract]
Property, plant and equipment	343,000,000		277,000,000
Amortization of intangible assets	14,000,000
Amortization of energy contract assets and liabilities	532,000,000	[1]
Nuclear fuel	419,000,000	[1]	355,000,000	[1]
Asset retirement obligation accretion	115,000,000	[2]	103,000,000	[2]
Total depreciation, amortization and accretion	1,423,000,000		735,000,000
Other Non-Cash Operating Activities [Abstract]
Pension and non-pension postretirement benefits costs	175,000,000		123,000,000
Provision for uncollectible accounts	(1,000,000)		0
Other Decommissioning Related Activity	(62,000,000)	[3]	(35,000,000)	[3]
Energy-related options	64,000,000	[4]	68,000,000	[4]
Amortization of debt fair value adjustment	(11,000,000)
Merger related commitments	35,000,000	[7]
Severance Costs	30,000,000
Gain (loss) on equity method investments	79,000,000
Other	9,000,000		12,000,000
Total other noncash operating activities	318,000,000		168,000,000
Changes In Other Assets and Liabilities [Abstract]
Other current assets and liabilities	(97,000,000)		(91,000,000)
Other noncurrent assets and liabilities	(77,000,000)		(17,000,000)
Total changes in other assets and liabilities	(174,000,000)		(108,000,000)
Cash Flow Noncash Investing And Financing Activities Disclosure [Abstract]
Merger with Constellation, common stock issued	5,272,000,000
Total noncash investing and financing activities	5,272,000,000
Commonwealth Edison Co [Member]
Depreciation, Amortization and Accretion [Abstract]
Property, plant and equipment	262,000,000		248,000,000
Regulatory assets	38,000,000		22,000,000
Total depreciation, amortization and accretion	300,000,000		270,000,000
Other Non-Cash Operating Activities [Abstract]
Pension and non-pension postretirement benefits costs	137,000,000		108,000,000
Provision for uncollectible accounts	21,000,000		18,000,000
Amortization of regulatory asset related to debt costs	7,000,000		9,000,000
Uncollectible accounts recovery			13,000,000
Discrete impacts from 2010 Rate Case Order			(32,000,000)	[5]
Discrete impacts from EIMA	69,000,000	[6]
Severance Costs	0
Other	7,000,000		(1,000,000)
Total other noncash operating activities	241,000,000		115,000,000
Changes In Other Assets and Liabilities [Abstract]
Under/over-recovered energy and transmission costs	(58,000,000)		(82,000,000)
Other regulatory assets and liabilities	39,000,000		6,000,000
Other current assets and liabilities	(1,000,000)		(13,000,000)
Other noncurrent assets and liabilities	(6,000,000)		139,000,000
Total changes in other assets and liabilities	(104,000,000)		38,000,000
PECO Energy Co [Member]
Depreciation, Amortization and Accretion [Abstract]
Property, plant and equipment	102,000,000		93,000,000
Regulatory assets	5,000,000		5,000,000
Total depreciation, amortization and accretion	107,000,000		98,000,000
Other Non-Cash Operating Activities [Abstract]
Pension and non-pension postretirement benefits costs	26,000,000		16,000,000
Provision for uncollectible accounts	26,000,000		27,000,000
Amortization of regulatory asset related to debt costs	1,000,000		2,000,000
Severance Costs	7,000,000
Other	6,000,000		(1,000,000)
Total other noncash operating activities	66,000,000		44,000,000
Changes In Other Assets and Liabilities [Abstract]
Under/over-recovered energy and transmission costs	18,000,000		(17,000,000)
Other regulatory assets and liabilities	(12,000,000)		(2,000,000)
Other current assets and liabilities	(110,000,000)	[8]	(104,000,000)
Other noncurrent assets and liabilities	(7,000,000)		11,000,000
Total changes in other assets and liabilities	(87,000,000)		(108,000,000)
DOE Smart Grid Investment Grant [Abstract]
Amount included in capital expenditures	34,000,000		19,000,000
Smart Grid Grant Reimbursements	46,000,000		26,000,000
Baltimore Gas and Electric Company [Member]
Depreciation, Amortization and Accretion [Abstract]
Property, plant and equipment	123,000,000		113,000,000
Regulatory assets	27,000,000		31,000,000
Total depreciation, amortization and accretion	150,000,000		144,000,000
Other Non-Cash Operating Activities [Abstract]
Pension and non-pension postretirement benefits costs	32,000,000		28,000,000
Provision for uncollectible accounts	17,000,000		21,000,000
Amortization of regulatory asset related to debt costs	1,000,000		1,000,000
Amortization of rate stabilization deferral	25,000,000		25,000,000
Deferral of storm costs			(16,000,000)
Merger related commitments	28,000,000	[7]
Severance Costs	0
Other	(3,000,000)		(5,000,000)
Total other noncash operating activities	100,000,000		54,000,000
Changes In Other Assets and Liabilities [Abstract]
Under/over-recovered energy and transmission costs	2,000,000		(7,000,000)
Other regulatory assets and liabilities	28,000,000		11,000,000
Other current assets and liabilities	55,000,000		28,000,000
Other noncurrent assets and liabilities	14,000,000		(7,000,000)
Total changes in other assets and liabilities	43,000,000		3,000,000
DOE Smart Grid Investment Grant [Abstract]
Amount included in capital expenditures	34,000,000		15,000,000
Smart Grid Grant Reimbursements	$ 19,000,000		$ 28,000,000

[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, for the 2011 annual reconciliation in ComEd's distribution formula rate tariff. This amount was partially offset by a reduction recorded in 2012 to the regulatory asset for the 2011 annual reconciliation previously established. 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.