Supplemental Financial Information (Details) (USD $)	3 Months Ended				12 Months Ended
	Mar. 31, 2012		Mar. 31, 2011		Dec. 31, 2010	Dec. 31, 2011
Accrued Liabilities Current [Abstract]
Total accrued expenses	$ 1,492,000,000					$ 1,255,000,000
Accumulated Other Comprehensive Income (Loss) [Abstract]
Net unrealized gain (loss) on cash flow hedges	703,000,000					488,000,000
Pension and non-pension postretirement benefit plans	(2,904,000,000)					(2,938,000,000)
Accumulated unrealized gain (loss) on marketable securities	2,000,000
Accumulated other comprehensive income (loss), net	(2,199,000,000)					(2,450,000,000)
Property, Plant And Equipment [Abstract]
Accumulated depreciation	(11,244,000,000)	[1]				(10,959,000,000)	[2]
Accounts receivable, net
Allowance for uncollectible accounts	(317,000,000)					(199,000,000)
Other Non-Cash Operating Activities [Abstract]
Pension and non-pension postretirement benefits costs	181,000,000		136,000,000
Provision for uncollectible accounts	40,000,000		45,000,000
Stock-based compensation costs	39,000,000		28,000,000
Other Decommissioning Related Activity	(71,000,000)	[3]	(32,000,000)	[3]
Energy-related options	28,000,000	[4]	34,000,000	[4]
Amortization of regulatory asset related to debt costs	6,000,000		6,000,000
Uncollectible accounts recovery			4,000,000
Discrete impacts from EIMA	38,000,000	[5]
FERC Investigation Settlement	0
Merger related commitments	331,000,000	[6]
Amortization of rate stabilization deferral	3,000,000
Equity in loss of unconsolidated subsidiaries	22,000,000
Other	(11,000,000)		2,000,000
Total other non-cash operating activities	530,000,000		223,000,000
Changes In Other Assets and Liabilities [Abstract]
Under/over-recovered energy and transmission costs	(25,000,000)		(60,000,000)
Other current assets and liabilities	(18,000,000)		(169,000,000)
Other regulatory assets and liabilities	(97,000,000)		3,000,000
Other noncurrent assets and liabilities	18,000,000		8,000,000
Total changes in other assets and liabilities	(122,000,000)		(218,000,000)
Cash Flow Noncash Investing And Financing Activities Disclosure [Abstract]
Change in ARC	80,000,000
Capital expenditures not paid	(81,000,000)
Contingent consideration at fair value	7,365,000,000
DOE Smart Grid Investment Grant [Abstract]
Amount included in capital expenditures	30,000,000		15,000,000
Smart Grid Grant Reimbursements	35,000,000		18,000,000
Depreciation, Amortization and Accretion [Abstract]
Property, plant and equipment	354,000,000		313,000,000
Regulatory assets	25,000,000		14,000,000
Nuclear fuel	207,000,000	[7]	174,000,000	[7]
Asset retirement obligation accretion	56,000,000	[8]	51,000,000	[8]
Amortization of intangible assets	3,000,000
Amortization of energy contract assets and liabilities	131,000,000
Total depreciation, amortization and accretion	776,000,000		552,000,000
Supplemental Financial Information Textuals [Abstract]
Payment to IRS					302,000,000
Operating revenues [Abstract]
Total operating revenues	4,686,000,000		4,956,000,000
Taxes Excluding Income And Excise Taxes [Abstract]
Total taxes other than income	194,000,000		203,000,000
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units	60,000,000	[9]	43,000,000	[9]
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units	48,000,000	[9]	10,000,000	[9]
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units	247,000,000		111,000,000
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement	65,000,000		43,000,000
Regulatory offset to decommissioning trust fund-related activities	(277,000,000)	[10]	(151,000,000)	[10]
Net unrealized income (losses) on pledged assets	35,000,000		23,000,000
Total decommissioning-related activities	178,000,000		79,000,000
Investment income	4,000,000		1,000,000
Long-term lease income	7,000,000		7,000,000
Interest income related to uncertain income tax positions	0		3,000,000
AFUDC - equity	3,000,000		5,000,000
Other	2,000,000		(1,000,000)
Other, net	194,000,000		94,000,000
Bargain purchase gain Wolf Hollow acquisition	0
Exelon Generation Co L L C [Member]
Capital Leases, Net Investment in Direct Financing Leases [Abstract]
Net investment in long-term leases	663,000,000
Accrued Liabilities Current [Abstract]
Total accrued expenses	894,000,000					779,000,000
Accumulated Other Comprehensive Income (Loss) [Abstract]
Net unrealized gain (loss) on cash flow hedges	1,167,000,000					915,000,000
Accumulated unrealized gain (loss) on marketable securities	0
Accumulated other comprehensive income (loss), net	1,167,000,000					915,000,000
Property, Plant And Equipment [Abstract]
Accumulated depreciation	(5,563,000,000)	[1]				(5,313,000,000)	[2]
Accounts receivable, net
Allowance for uncollectible accounts	(82,000,000)					(29,000,000)
Other Non-Cash Operating Activities [Abstract]
Pension and non-pension postretirement benefits costs	81,000,000		62,000,000
Provision for uncollectible accounts	(9,000,000)		0
Other Decommissioning Related Activity	(71,000,000)	[3]	(32,000,000)	[3]
Energy-related options	28,000,000	[4]	34,000,000	[4]
FERC Investigation Settlement	0
Merger related commitments	35,000,000
Equity in loss of unconsolidated subsidiaries	22,000,000
Other	4,000,000		8,000,000
Total other non-cash operating activities	90,000,000		72,000,000
Changes In Other Assets and Liabilities [Abstract]
Other current assets and liabilities	(122,000,000)		(20,000,000)
Other noncurrent assets and liabilities	42,000,000		0
Total changes in other assets and liabilities	(80,000,000)		(20,000,000)
Cash Flow Noncash Investing And Financing Activities Disclosure [Abstract]
Change in ARC	80,000,000
Capital expenditures not paid	(52,000,000)
Contingent consideration at fair value	5,272,000,000
Depreciation, Amortization and Accretion [Abstract]
Property, plant and equipment	150,000,000		139,000,000
Nuclear fuel	207,000,000	[7]	174,000,000	[7]
Asset retirement obligation accretion	56,000,000	[8]	51,000,000	[8]
Amortization of intangible assets	3,000,000
Amortization of energy contract assets and liabilities	131,000,000
Total depreciation, amortization and accretion	547,000,000		364,000,000
Operating revenues [Abstract]
Total operating revenues	2,739,000,000		2,643,000,000
Taxes Excluding Income And Excise Taxes [Abstract]
Total taxes other than income	73,000,000		66,000,000
Decommissioning-Related Activities [Abstract]
Net realized income on decommissioning trust funds - Regulatory Agreement Units	60,000,000	[9]	43,000,000	[9]
Net realized income on decommissioning trust funds - Non-Regulatory Agreement Units	48,000,000	[9]	10,000,000	[9]
Net unrealized income (losses) on decommissioning trust funds - Regulatory Agreement Units	247,000,000		111,000,000
Net unrealized income (losses) on decommissioning trust funds - Non-Regulatory Agreement	65,000,000		43,000,000
Regulatory offset to decommissioning trust fund-related activities	(277,000,000)	[10]	(151,000,000)	[10]
Net unrealized income (losses) on pledged assets	35,000,000		23,000,000
Total decommissioning-related activities	178,000,000		79,000,000
Investment income	0		0
Interest income related to uncertain income tax positions	0		0
Other	0		(4,000,000)
Other, net	178,000,000		75,000,000
Bargain purchase gain Wolf Hollow acquisition	0
Commonwealth Edison Co [Member]
Accrued Liabilities Current [Abstract]
Total accrued expenses	204,000,000					318,000,000
Accumulated Other Comprehensive Income (Loss) [Abstract]
Net unrealized gain (loss) on cash flow hedges						0
Accumulated unrealized gain (loss) on marketable securities	0					(1,000,000)
Accumulated other comprehensive income (loss), net	0					(1,000,000)
Property, Plant And Equipment [Abstract]
Accumulated depreciation	(2,769,000,000)					(2,750,000,000)
Accounts receivable, net
Allowance for uncollectible accounts	(90,000,000)					(78,000,000)
Other Non-Cash Operating Activities [Abstract]
Pension and non-pension postretirement benefits costs	69,000,000		54,000,000
Provision for uncollectible accounts	22,000,000		19,000,000
Amortization of regulatory asset related to debt costs	4,000,000		5,000,000
Uncollectible accounts recovery			4,000,000
Discrete impacts from EIMA	38,000,000	[5]
Other	3,000,000		0
Total other non-cash operating activities	60,000,000		82,000,000
Changes In Other Assets and Liabilities [Abstract]
Under/over-recovered energy and transmission costs	(38,000,000)		(29,000,000)
Other current assets and liabilities	1,000,000		(8,000,000)
Other regulatory assets and liabilities	(16,000,000)		5,000,000
Other noncurrent assets and liabilities	(19,000,000)		(3,000,000)
Total changes in other assets and liabilities	(72,000,000)		(35,000,000)
Cash Flow Noncash Investing And Financing Activities Disclosure [Abstract]
Capital expenditures not paid	(14,000,000)
Depreciation, Amortization and Accretion [Abstract]
Property, plant and equipment	131,000,000		122,000,000
Regulatory assets	18,000,000		12,000,000
Total depreciation, amortization and accretion	149,000,000		134,000,000
Operating revenues [Abstract]
Total operating revenues	1,388,000,000		1,466,000,000
Taxes Excluding Income And Excise Taxes [Abstract]
Total taxes other than income	75,000,000		77,000,000
Decommissioning-Related Activities [Abstract]
Investment income	1,000,000		0
Interest income related to uncertain income tax positions	0		1,000,000
AFUDC - equity	1,000,000		1,000,000
Other	2,000,000		2,000,000
Other, net	4,000,000		4,000,000
PECO Energy Co [Member]
Accrued Liabilities Current [Abstract]
Total accrued expenses	78,000,000					83,000,000
Accumulated Other Comprehensive Income (Loss) [Abstract]
Net unrealized gain (loss) on cash flow hedges						0
Accumulated unrealized gain (loss) on marketable securities	1,000,000
Accumulated other comprehensive income (loss), net	1,000,000					0
Financing Receivable Recorded Investment [Line Items]
Installment plan receivables	17,000,000					21,000,000
Installment plan receivables uncollectible accounts reserve	(13,000,000)					(17,000,000)
Property, Plant And Equipment [Abstract]
Accumulated depreciation	(2,691,000,000)					(2,662,000,000)
Accounts receivable, net
Allowance for uncollectible accounts	(111,000,000)					(92,000,000)
Other Non-Cash Operating Activities [Abstract]
Pension and non-pension postretirement benefits costs	13,000,000		8,000,000
Provision for uncollectible accounts	24,000,000		26,000,000
Amortization of regulatory asset related to debt costs	1,000,000		1,000,000
Other	2,000,000		0
Total other non-cash operating activities	40,000,000		35,000,000
Changes In Other Assets and Liabilities [Abstract]
Under/over-recovered energy and transmission costs	13,000,000		(31,000,000)
Other current assets and liabilities	(134,000,000)	[11]	(136,000,000)
Other regulatory assets and liabilities	3,000,000		(2,000,000)
Other noncurrent assets and liabilities	8,000,000		7,000,000
Total changes in other assets and liabilities	(110,000,000)		(162,000,000)
Cash Flow Noncash Investing And Financing Activities Disclosure [Abstract]
Capital expenditures not paid	12,000,000
DOE Smart Grid Investment Grant [Abstract]
Amount included in capital expenditures	20,000,000		9,000,000
Smart Grid Grant Reimbursements	21,000,000		5,000,000
Depreciation, Amortization and Accretion [Abstract]
Property, plant and equipment	51,000,000		46,000,000
Regulatory assets	2,000,000		2,000,000
Total depreciation, amortization and accretion	53,000,000		48,000,000
Operating revenues [Abstract]
Total operating revenues	875,000,000		1,153,000,000
Taxes Excluding Income And Excise Taxes [Abstract]
Total taxes other than income	31,000,000		56,000,000
Decommissioning-Related Activities [Abstract]
Investment income	1,000,000		1,000,000
Interest income related to uncertain income tax positions	0		0
AFUDC - equity	1,000,000		4,000,000
Other	0		1,000,000
Other, net	2,000,000		6,000,000
PECO Energy Co [Member] | Low Risk [Member]
Financing Receivable Recorded Investment [Line Items]
Installment plan receivables uncollectible accounts reserve	0					0
PECO Energy Co [Member] | Low To Medium Risk [Member]
Financing Receivable Recorded Investment [Line Items]
Installment plan receivables uncollectible accounts reserve	(1,000,000)					(1,000,000)
PECO Energy Co [Member] | Medium Risk [Member]
Financing Receivable Recorded Investment [Line Items]
Installment plan receivables uncollectible accounts reserve	(4,000,000)					(3,000,000)
PECO Energy Co [Member] | High Risk [Member]
Financing Receivable Recorded Investment [Line Items]
Installment plan receivables uncollectible accounts reserve	(8,000,000)					(13,000,000)
Baltimore Gas and Electric Company [Member]
Accrued Liabilities Current [Abstract]
Total accrued expenses	113,000,000					148,000,000
Accumulated Other Comprehensive Income (Loss) [Abstract]
Net unrealized gain (loss) on cash flow hedges						0
Accumulated unrealized gain (loss) on marketable securities	0
Accumulated other comprehensive income (loss), net	0					0
Property, Plant And Equipment [Abstract]
Accumulated depreciation	(2,508,000,000)					(2,465,000,000)
Accounts receivable, net
Allowance for uncollectible accounts	(34,000,000)					(38,000,000)
Other Non-Cash Operating Activities [Abstract]
Pension and non-pension postretirement benefits costs	16,000,000		11,000,000
Provision for uncollectible accounts	8,000,000		9,000,000
Amortization of regulatory asset related to debt costs	1,000,000		1,000,000
Merger related commitments	141,000,000	[6]
Amortization of rate stabilization deferral	13,000,000		14,000,000
Deferral of storm costs			(16,000,000)
Other	(1,000,000)		(3,000,000)
Total other non-cash operating activities	178,000,000		16,000,000
Changes In Other Assets and Liabilities [Abstract]
Under/over-recovered energy and transmission costs	(10,000,000)		3,000,000
Other current assets and liabilities	31,000,000		31,000,000
Other regulatory assets and liabilities	(16,000,000)		(3,000,000)
Other noncurrent assets and liabilities	7,000,000		(5,000,000)
Total changes in other assets and liabilities	12,000,000		26,000,000
DOE Smart Grid Investment Grant [Abstract]
Amount included in capital expenditures	10,000,000		6,000,000
Smart Grid Grant Reimbursements	14,000,000		13,000,000
Depreciation, Amortization and Accretion [Abstract]
Property, plant and equipment	61,000,000		57,000,000
Regulatory assets	18,000,000		20,000,000
Total depreciation, amortization and accretion	79,000,000		77,000,000
Operating revenues [Abstract]
Total operating revenues	696,000,000		976,000,000
Taxes Excluding Income And Excise Taxes [Abstract]
Total taxes other than income	48,000,000		50,000,000
Decommissioning-Related Activities [Abstract]
Investment income	3,000,000		4,000,000
Interest income related to uncertain income tax positions	0		0
AFUDC - equity	3,000,000		4,000,000
Other	0		0
Other, net	$ 6,000,000		$ 8,000,000

[1] Includes accumulated amortization of nuclear fuel in the reactor core of $1,851 million.

[2] Includes accumulated amortization of nuclear fuel in the reactor core of $1,784 million.

[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] Includes the establishment of a regulatory asset, pursuant to EIMA, for the 2012 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.

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

[7] Included in purchased power and fuel expense on the Registrants' Consolidated Statements of Operations and Comprehensive Income.

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

[9] Includes investment income and realized gains and losses on sales of investments of the trust funds.

[10] Includes the elimination of NDT fund-related activity for the Regulatory Agreement Units, including the elimination of net realized income taxes related to all NDT fund activity for those units. See Note 12 of the Exelon 2011 Form 10-K for additional information regarding the accounting for nuclear decommissioning.

[11] Relates primarily to prepaid utility taxes.