Supplemental Financial Information - Cash Flow (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended						37 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2012
Depreciation, Amortization and Accretion [Abstract]
Property, plant and equipment	$ 1,712.00		$ 1,284.00		$ 1,144.00
Regulatory assets	129.00		63.00		931.00
Amortization of intangible assets, net	40.00
Amortization of energy contract assets and liabilities	1,110.00
Nuclear fuel	848.00	[1]	755.00	[1]	672.00	[1]
Asset retirement obligation accretion	240.00	[2]	214.00	[2]	196.00	[2]
Total depreciation, amortization and accretion	4,079.00		2,316.00		2,943.00
Cash Paid Refunded During Year [Abstract]
Interest (net of amount capitalized)	761.00		649.00		665.00	[3]
Income taxes (net of refunds)	(171.00)		(457.00)		1,219.00
Other Non-Cash Operating Activities [Abstract]
Pension and non-pension postretirement benefits costs	820.00		542.00		581.00
Provision for uncollectible accounts	164.00		121.00		108.00
Provision for Obsolete Inventory	6.00
Stock-based compensation costs	94.00		67.00		44.00
Other Decommissioning Related Activity	145.00		(16.00)	[4]	91.00	[5]
Energy-related options	160.00		137.00	[6]	(73.00)	[3]
ARO adjustment			0		(19.00)
Amortization of regulatory asset related to debt costs	18.00		21.00		24.00
Amortization of rate stabilization deferral	57.00		57.00
Amortization of debt fair value adjustment	34.00
Accrual for Illinois utility distribution tax refund					(25.00)
APR SO2 allowances impairment			0		(57.00)
Discrete impacts from 2010 Rate Case Order			32.00
Discrete impacts from EIMA	96.00		82.00	[7]
Merger related commitments	(141.00)
Severance Costs	(99.00)
Gain (loss) on equity method investments	91.00		1.00		0
Amortization of debt costs	(19.00)
Other	(11.00)		2.00		5.00
Total other noncash operating activities	(1,383.00)
Changes In Other Assets and Liabilities [Abstract]
Under/over-recovered energy and transmission costs	71.00		(45.00)		61.00
Other regulatory assets and liabilities	(404.00)
Other current assets and liabilities	213.00
Other noncurrent assets and liabilities	(248.00)		122.00		(99.00)
Total changes in other assets and liabilities	(368.00)		(24.00)		(56.00)
Cash Flow Noncash Investing And Financing Activities Disclosure [Abstract]
Change in ARC	781.00		186.00		(428.00)
Capital expenditures not paid	160.00		96.00		34.00
Purchase accounting adjustments			0		9.00
Merger with Constellation, common stock issued	7,365.00
DOE Smart Grid Investment Grant [Abstract]
Amount included in capital expenditures	103.00		51.00
Smart Grid Grant Reimbursements	113.00		56.00
Exelon Generation Co L L C [Member]
Depreciation, Amortization and Accretion [Abstract]
Property, plant and equipment	733.00		570.00		474.00
Regulatory assets	0		0		0
Amortization of intangible assets, net	35.00
Amortization of energy contract assets and liabilities	1,110.00
Nuclear fuel	848.00	[1]	755.00	[1]	672.00	[1]
Asset retirement obligation accretion	240.00		214.00	[2]	195.00	[2]
Total depreciation, amortization and accretion	2,966.00		1,539.00		1,341.00
Cash Paid Refunded During Year [Abstract]
Interest (net of amount capitalized)	286.00		158.00		145.00
Income taxes (net of refunds)	175.00		347.00		732.00
Other Non-Cash Operating Activities [Abstract]
Pension and non-pension postretirement benefits costs	341.00		249.00		268.00
Provision for uncollectible accounts	22.00		0		1.00
Provision for Obsolete Inventory	6.00						18.00
Other Decommissioning Related Activity	145.00		(16.00)	[4]	91.00	[5]
Energy-related options	160.00		137.00	[6]	(73.00)	[3]
ARO adjustment			0		(8.00)
Amortization of debt fair value adjustment	34.00
APR SO2 allowances impairment			0		(57.00)
Merger related commitments	(32.00)
Severance Costs	(34.00)
Gain (loss) on equity method investments	91.00		1.00		0
Amortization of debt costs	(11.00)
Other	19.00		55.00		16.00
Total other noncash operating activities	(537.00)
Changes In Other Assets and Liabilities [Abstract]
Other current assets and liabilities	(30.00)
Other noncurrent assets and liabilities	(98.00)		(34.00)		(29.00)
Total changes in other assets and liabilities	(128.00)		(57.00)		(45.00)
Cash Flow Noncash Investing And Financing Activities Disclosure [Abstract]
Change in ARC	781.00		186.00		(428.00)
Capital expenditures not paid	103.00		125.00		13.00
Purchase accounting adjustments			0		9.00
Merger with Constellation, common stock issued	5,264.00
Commonwealth Edison Co [Member]
Depreciation, Amortization and Accretion [Abstract]
Property, plant and equipment	525.00		502.00		473.00
Regulatory assets	80.00		52.00		43.00
Amortization of intangible assets, net	5.00
Nuclear fuel	0		0		0
Asset retirement obligation accretion	0		0	[2]	1.00	[2]
Total depreciation, amortization and accretion	610.00		554.00		517.00
Cash Paid Refunded During Year [Abstract]
Interest (net of amount capitalized)	288.00		296.00		283.00
Income taxes (net of refunds)	(42.00)		(676.00)		15.00
Other Non-Cash Operating Activities [Abstract]
Pension and non-pension postretirement benefits costs	282.00		213.00		215.00
Provision for uncollectible accounts	42.00		57.00		48.00
Provision for Obsolete Inventory	1.00
ARO adjustment			0		(10.00)
Amortization of regulatory asset related to debt costs	13.00		18.00		20.00
Accrual for Illinois utility distribution tax refund					(25.00)
Discrete impacts from 2010 Rate Case Order			32.00
Discrete impacts from EIMA	96.00		82.00	[7]
Gain (loss) on equity method investments	0		0		0
Amortization of debt costs	(5.00)
Other	5.00		(4.00)		4.00
Total other noncash operating activities	(252.00)
Changes In Other Assets and Liabilities [Abstract]
Under/over-recovered energy and transmission costs	28.00		(49.00)		58.00
Other regulatory assets and liabilities	(68.00)		44.00		(19.00)
Other current assets and liabilities	(7.00)
Other noncurrent assets and liabilities	(95.00)		64.00		(184.00)	[8]
Total changes in other assets and liabilities	(142.00)		45.00		(133.00)
Cash Flow Noncash Investing And Financing Activities Disclosure [Abstract]
Change in ARC	2.00
Capital expenditures not paid	15.00		7.00		7.00
PECO Energy Co [Member]
Depreciation, Amortization and Accretion [Abstract]
Property, plant and equipment	207.00		191.00		171.00
Regulatory assets	10.00		11.00	[9]	889.00	[9]
Nuclear fuel	0		0		0
Asset retirement obligation accretion	0		0	[2]	0	[2]
Total depreciation, amortization and accretion	217.00		202.00		1,060.00
Cash Paid Refunded During Year [Abstract]
Interest (net of amount capitalized)	113.00		128.00		168.00
Income taxes (net of refunds)	(64.00)		(65.00)		433.00
Other Non-Cash Operating Activities [Abstract]
Pension and non-pension postretirement benefits costs	50.00		32.00		46.00
Provision for uncollectible accounts	60.00		64.00		59.00
ARO adjustment			0		(1.00)
Amortization of regulatory asset related to debt costs	3.00		3.00		4.00
Amortization of debt costs	(3.00)
Other	9.00		1.00		0
Total other noncash operating activities	(125.00)
Changes In Other Assets and Liabilities [Abstract]
Under/over-recovered energy and transmission costs	20.00		4.00		3.00
Other regulatory assets and liabilities	18.00		26.00		35.00
Other current assets and liabilities	(12.00)
Other noncurrent assets and liabilities	(10.00)		(4.00)		59.00
Total changes in other assets and liabilities	16.00		14.00		78.00
Cash Flow Noncash Investing And Financing Activities Disclosure [Abstract]
Capital expenditures not paid	26.00		(35.00)		14.00
DOE Smart Grid Investment Grant [Abstract]
Amount included in capital expenditures	56.00		51.00
Smart Grid Grant Reimbursements	66.00		56.00
Baltimore Gas and Electric Company [Member]
Depreciation, Amortization and Accretion [Abstract]
Property, plant and equipment	245.00		224.00		214.00
Regulatory assets	53.00		50.00		35.00
Total depreciation, amortization and accretion	298.00		274.00		249.00
Cash Paid Refunded During Year [Abstract]
Interest (net of amount capitalized)	136.00		122.00		128.00
Income taxes (net of refunds)	(112.00)		(54.00)		(76.00)
Other Non-Cash Operating Activities [Abstract]
Pension and non-pension postretirement benefits costs	57.00		51.00		48.00
Provision for uncollectible accounts	44.00		44.00		38.00
Amortization of regulatory asset related to debt costs	2.00		2.00		2.00
Amortization of rate stabilization deferral	67.00		57.00		62.00
Deferral of storm costs	(16.00)		16.00
Merger related commitments	(27.00)
Amortization of debt costs	(2.00)
Other	(6.00)		(9.00)		(6.00)
Total other noncash operating activities	(193.00)		(129.00)		(144.00)
Changes In Other Assets and Liabilities [Abstract]
Under/over-recovered energy and transmission costs	26.00		(52.00)		6.00
Other regulatory assets and liabilities	(112.00)		10.00		(64.00)
Other current assets and liabilities	(7.00)		(88.00)		(7.00)
Other noncurrent assets and liabilities	8.00		(31.00)		2.00
Total changes in other assets and liabilities	(85.00)		(161.00)		(63.00)
Cash Flow Noncash Investing And Financing Activities Disclosure [Abstract]
Capital expenditures not paid	(4.00)		(7.00)		28.00
DOE Smart Grid Investment Grant [Abstract]
Amount included in capital expenditures	47.00		23.00
Smart Grid Grant Reimbursements	$ 47.00		$ 41.00

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

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

[3] Includes the elimination of NDT fund 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 - Asset Retirement Obligations for additional information regarding the accounting for nuclear decommissioning.

[4] Includes amounts reclassified to realized at settlement of contracts recorded to results of operations related to option premiums due to the settlement of underlying transactions.

[5] Excludes $167 million of interest paid to the IRS relating to a preliminary agreement reached during the third quarter of 2010. See Note 12 - Income Taxes for addition information.

[6] In May 2011, as a result of the 2010 Rate Case order, ComEd recorded one-time benefits to reestablish previously expensed plant balances and to recover previously incurred costs related to Exelon's 2009 restructuring plan. See Note 3 - Regulatory Matters for more information.

[7] Includes the establishment of a regulatory asset, pursuant to EIMA, for the 2011 annual reconciliation in ComEd's distribution formula rate tariff and the deferral of costs associated with significant 2011 storms, partially offset by an accrual to fund a new Science and Technology Innovation Trust. See Note 3 - Regulatory Matters for more information.

[8] Relates primarily to a decrease in interest payable associated with a change in uncertain income tax positions. See Note 12 - Income Taxes for additional information.

[9] For PECO, primarily reflects CTC amortization.