Fair Value of Financial Assets and Liabilities (Fair Value Measurements, Recurring and Nonrecurring) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012		Mar. 31, 2012
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	$ 383		$ 995	[1]
Fixed income [Abstract]
Other investments	11		19
Deferred compensation	(101)		(102)
Total assets	9,946		10,785
Total liabilities	(541)		(735)
Total net assets	9,405		10,050
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Deferred compensation	101		102
Mark-to-market derivative liabilities (current liabilities)	181		352
Mark-to-market derivative liabilities (noncurrent liabilities)	259		281
Nuclear Decommissioning Trust Fund Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	216		245
Equity Securities [Abstract]
Equity securities	1,649		1,480
Commingled funds	2,073		1,933
Equity securities subtotal	3,722		3,413
Fixed income [Abstract]
Debt securities issued by the U.S. Treasury and other U.S. government corporations and agencies	1,039		1,057
Debt securities issued by states of the United States and political subdivisions of the states	328		321
Debt securities issued by foreign governments	108		93
Corporate debt securities	1,796		1,788
Federal agency mortgage-backed securities	23		24
Commercial mortgage-backed securities (non-agency)	43		45
Residential mortgage-backed securities (non-agency)	11		11
Mutual funds fixed income	26		23
Fixed income subtotal	3,374		3,362
Direct lending securities	210		183
Other debt obligations	16		15
Nuclear decommissioning trust fund investments subtotal	7,538	[2]	7,218	[2]
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Net assets (liabilities) excluded from nuclear decommissioning trust fund investments	21		30
Pledged Assets For Zion Station Decommissioning [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	41		23
Equity Securities [Abstract]
Equity securities			14
Commingled funds			9
Equity securities subtotal			23
Fixed income [Abstract]
Debt securities issued by the U.S. Treasury and other U.S. government corporations and agencies	148		130
Debt securities issued by states of the United States and political subdivisions of the states	29		37
Corporate debt securities	224		249
Federal agency mortgage-backed securities	29		49
Commercial mortgage-backed securities (non-agency)			6
Fixed income subtotal	430		471
Direct lending securities	104		89
Other debt obligations			1
Pledged assets for Zion Station decommissioning subtotal	575	[3]	607	[3]
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Net assets (liabilities) excluded from pledged assets	5		7
Rabbi Trust Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	1		2
Fixed income [Abstract]
Mutual funds	66	[4],[5]	69	[4],[5]
Rabbi trust investments subtotal	67		71
Deferred compensation	(51)		(53)
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Deferred compensation	51		53
Supplemental executive retirement plan fair value	15		16
Cash surrender value of life insurance investments excluded from Rabbi Trust investments	29		28
Derivative Financial Instruments Liabilities [Member]
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Fair value of energy swap contract current liability	15		18
Fair value of energy swap contract noncurrent liability	60		49
Commodity Mark To Market Derivative Assets [Member]
Fixed income [Abstract]
Other derivatives	3,890		4,675
Proprietary trading	2,577		3,193
Effect of netting and allocation of collateral received/(paid)	(5,152)	[6]	(6,056)	[6]
Mark-to-market subtotal	1,315	[7]	1,812	[7]
Commodity Mark To Market Derivative Liabilities [Member]
Fixed income [Abstract]
Other derivatives	(3,298)		(3,566)
Proprietary trading	(2,499)		(3,121)
Effect of netting and allocation of collateral received/(paid)	5,385	[6]	6,087	[6]
Mark-to-market subtotal	(412)	[7],[8]	(600)	[7],[8]
Interest Rate Mark To Market Derivative Assets [Member]
Fixed income [Abstract]
Effect of netting and allocation of collateral received/(paid)	43		51
Interest rate mark to market	100		114
Interest rate mark-to-market Subtotal	57		63
Interest Rate Mark To Market Derivative Liabilities [Member]
Fixed income [Abstract]
Effect of netting and allocation of collateral received/(paid)	(43)		(51)
Interest rate mark to market	(71)		(84)
Interest rate mark-to-market Subtotal	(28)		(33)
Fair Value Inputs Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	383		995	[1]
Fixed income [Abstract]
Other investments	2		2
Deferred compensation	0
Total assets	3,631		4,062
Total liabilities	(69)		(83)
Total net assets	3,562		3,979
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Deferred compensation	0
Collateral received from counterparties, net of collateral paid to counterparties	169		219
Fair Value Inputs Level 1 [Member] | Nuclear Decommissioning Trust Fund Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	216		245
Equity Securities [Abstract]
Equity securities	1,649		1,480
Commingled funds	0
Equity securities subtotal	1,649		1,480
Fixed income [Abstract]
Debt securities issued by the U.S. Treasury and other U.S. government corporations and agencies	1,039		1,057
Debt securities issued by states of the United States and political subdivisions of the states	0
Debt securities issued by foreign governments	0
Corporate debt securities	0
Federal agency mortgage-backed securities	0
Commercial mortgage-backed securities (non-agency)	0
Residential mortgage-backed securities (non-agency)	0
Mutual funds fixed income	0
Fixed income subtotal	1,039		1,057
Direct lending securities	0
Other debt obligations	0
Nuclear decommissioning trust fund investments subtotal	2,904	[2]	2,782	[2]
Fair Value Inputs Level 1 [Member] | Pledged Assets For Zion Station Decommissioning [Member]
Equity Securities [Abstract]
Equity securities			14
Equity securities subtotal			14
Fixed income [Abstract]
Debt securities issued by the U.S. Treasury and other U.S. government corporations and agencies	138		118
Debt securities issued by states of the United States and political subdivisions of the states	0
Corporate debt securities	0
Federal agency mortgage-backed securities	0
Fixed income subtotal	138		118
Direct lending securities	0
Pledged assets for Zion Station decommissioning subtotal	138	[3]	132	[3]
Fair Value Inputs Level 1 [Member] | Rabbi Trust Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	1		2
Fixed income [Abstract]
Mutual funds	66	[4],[5]	69	[4],[5]
Rabbi trust investments subtotal	67		71
Fair Value Inputs Level 1 [Member] | Commodity Mark To Market Derivative Assets [Member]
Fixed income [Abstract]
Other derivatives	801		861
Proprietary trading	764		1,042
Effect of netting and allocation of collateral received/(paid)	(1,428)	[6]	(1,823)	[6]
Mark-to-market subtotal	137	[7]	80	[7]
Fair Value Inputs Level 1 [Member] | Commodity Mark To Market Derivative Liabilities [Member]
Fixed income [Abstract]
Other derivatives	(884)		(1,041)
Proprietary trading	(782)		(1,084)
Effect of netting and allocation of collateral received/(paid)	1,597	[6]	2,042	[6]
Mark-to-market subtotal	(69)	[7],[8]	(83)	[7],[8]
Fair Value Inputs Level 1 [Member] | Interest Rate Mark To Market Derivative Assets [Member]
Fixed income [Abstract]
Interest rate mark to market	0
Fair Value Inputs Level 1 [Member] | Interest Rate Mark To Market Derivative Liabilities [Member]
Fixed income [Abstract]
Interest rate mark to market	0
Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	0		0	[1]
Fixed income [Abstract]
Deferred compensation	(101)		(102)
Total assets	5,834		5,778
Total liabilities	(574)		(363)
Total net assets	5,260		5,415
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Deferred compensation	101		102
Collateral received from counterparties, net of collateral paid to counterparties	64		(155)
Fair Value Inputs Level 2 [Member] | Nuclear Decommissioning Trust Fund Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	0
Equity Securities [Abstract]
Equity securities	0
Commingled funds	2,073		1,933
Equity securities subtotal	2,073		1,933
Fixed income [Abstract]
Debt securities issued by the U.S. Treasury and other U.S. government corporations and agencies	0
Debt securities issued by states of the United States and political subdivisions of the states	328		321
Debt securities issued by foreign governments	108		93
Corporate debt securities	1,796		1,788
Federal agency mortgage-backed securities	23		24
Commercial mortgage-backed securities (non-agency)	43		45
Residential mortgage-backed securities (non-agency)	11		11
Mutual funds fixed income	26		23
Fixed income subtotal	2,335		2,305
Direct lending securities	0
Other debt obligations	16		15
Nuclear decommissioning trust fund investments subtotal	4,424	[2]	4,253	[2]
Fair Value Inputs Level 2 [Member] | Pledged Assets For Zion Station Decommissioning [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	41		23
Equity Securities [Abstract]
Commingled funds			9
Equity securities subtotal			9
Fixed income [Abstract]
Debt securities issued by the U.S. Treasury and other U.S. government corporations and agencies	10		12
Debt securities issued by states of the United States and political subdivisions of the states	29		37
Corporate debt securities	224		249
Federal agency mortgage-backed securities	29		49
Commercial mortgage-backed securities (non-agency)			6
Fixed income subtotal	292		353
Direct lending securities	0
Other debt obligations			1
Pledged assets for Zion Station decommissioning subtotal	333	[3]	386	[3]
Fair Value Inputs Level 2 [Member] | Rabbi Trust Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	0
Fixed income [Abstract]
Mutual funds	0	[4],[5]
Rabbi trust investments subtotal	0
Fair Value Inputs Level 2 [Member] | Derivative Financial Instruments Assets [Member]
Fixed income [Abstract]
Other investments	0
Fair Value Inputs Level 2 [Member] | Commodity Mark To Market Derivative Assets [Member]
Fixed income [Abstract]
Other derivatives	2,535		3,173
Proprietary trading	1,637		2,078
Effect of netting and allocation of collateral received/(paid)	(3,152)	[6]	(4,175)	[6]
Mark-to-market subtotal	1,020	[7]	1,076	[7]
Fair Value Inputs Level 2 [Member] | Commodity Mark To Market Derivative Liabilities [Member]
Fixed income [Abstract]
Other derivatives	(2,131)		(2,289)
Proprietary trading	(1,530)		(1,959)
Effect of netting and allocation of collateral received/(paid)	3,216	[6]	4,020	[6]
Mark-to-market subtotal	(445)	[7],[8]	(228)	[7],[8]
Fair Value Inputs Level 2 [Member] | Interest Rate Mark To Market Derivative Assets [Member]
Fixed income [Abstract]
Effect of netting and allocation of collateral received/(paid)	43		51
Interest rate mark to market	100		114
Interest rate mark-to-market Subtotal	57		63
Fair Value Inputs Level 2 [Member] | Interest Rate Mark To Market Derivative Liabilities [Member]
Fixed income [Abstract]
Effect of netting and allocation of collateral received/(paid)	(43)		(51)
Interest rate mark to market	(71)		(84)
Interest rate mark-to-market Subtotal	(28)		(33)
Fair Value Inputs Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	0		0	[1]
Fixed income [Abstract]
Other investments	9		17
Deferred compensation	0
Total assets	481		945
Total liabilities	102		(289)
Total net assets	583		656
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Deferred compensation	0
Collateral received from counterparties, net of collateral paid to counterparties	0		(33)
Fair Value Inputs Level 3 [Member] | Nuclear Decommissioning Trust Fund Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	0
Equity Securities [Abstract]
Equity securities	0
Commingled funds	0
Equity securities subtotal	0
Fixed income [Abstract]
Debt securities issued by the U.S. Treasury and other U.S. government corporations and agencies	0
Debt securities issued by states of the United States and political subdivisions of the states	0
Debt securities issued by foreign governments	0
Corporate debt securities	0
Federal agency mortgage-backed securities	0
Commercial mortgage-backed securities (non-agency)	0
Residential mortgage-backed securities (non-agency)	0
Mutual funds fixed income	0
Fixed income subtotal	0
Direct lending securities	210		183
Other debt obligations	0
Nuclear decommissioning trust fund investments subtotal	210	[2]	183	[2]
Fair Value Inputs Level 3 [Member] | Pledged Assets For Zion Station Decommissioning [Member]
Fixed income [Abstract]
Debt securities issued by the U.S. Treasury and other U.S. government corporations and agencies	0
Debt securities issued by states of the United States and political subdivisions of the states	0
Corporate debt securities	0
Federal agency mortgage-backed securities	0
Fixed income subtotal	0
Direct lending securities	104		89
Pledged assets for Zion Station decommissioning subtotal	104	[3]	89	[3]
Fair Value Inputs Level 3 [Member] | Rabbi Trust Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	0
Fixed income [Abstract]
Mutual funds	0	[4],[5]
Rabbi trust investments subtotal	0
Fair Value Inputs Level 3 [Member] | Commodity Mark To Market Derivative Assets [Member]
Fixed income [Abstract]
Other derivatives	554		641
Proprietary trading	176		73
Effect of netting and allocation of collateral received/(paid)	(572)	[6]	(58)	[6]
Mark-to-market subtotal	158	[7]	656	[7]
Fair Value Inputs Level 3 [Member] | Commodity Mark To Market Derivative Liabilities [Member]
Fixed income [Abstract]
Other derivatives	(283)		(236)
Proprietary trading	(187)		(78)
Effect of netting and allocation of collateral received/(paid)	572	[6]	25	[6]
Mark-to-market subtotal	102	[7],[8]	(289)	[7],[8]
Fair Value Inputs Level 3 [Member] | Interest Rate Mark To Market Derivative Assets [Member]
Fixed income [Abstract]
Interest rate mark to market	0
Fair Value Inputs Level 3 [Member] | Interest Rate Mark To Market Derivative Liabilities [Member]
Fixed income [Abstract]
Interest rate mark to market	0
Exelon Generation Co L L C [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	56		487	[1]
Fixed income [Abstract]
Other investments	11		19
Deferred compensation	(26)		(28)
Total assets	9,637		10,433
Total liabilities	(391)		(594)
Total net assets	9,246		9,839
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Deferred compensation	26		28
Mark-to-market derivative liabilities (current liabilities)	166		334
Mark-to-market derivative liabilities (noncurrent liabilities)	199		232
Exelon Generation Co L L C [Member] | Nuclear Decommissioning Trust Fund Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	216		245
Equity Securities [Abstract]
Equity securities	1,649		1,480
Commingled funds	2,073		1,933
Equity securities subtotal	3,722		3,413
Fixed income [Abstract]
Debt securities issued by the U.S. Treasury and other U.S. government corporations and agencies	1,039		1,057
Debt securities issued by states of the United States and political subdivisions of the states	328		321
Debt securities issued by foreign governments	108		93
Corporate debt securities	1,796		1,788
Federal agency mortgage-backed securities	23		24
Commercial mortgage-backed securities (non-agency)	43		45
Residential mortgage-backed securities (non-agency)	11		11
Mutual funds fixed income	26		23
Fixed income subtotal	3,374		3,362
Direct lending securities	210		183
Other debt obligations	16		15
Nuclear decommissioning trust fund investments subtotal	7,538	[2]	7,218	[2]
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Net assets (liabilities) excluded from nuclear decommissioning trust fund investments	21		30
Exelon Generation Co L L C [Member] | Pledged Assets For Zion Station Decommissioning [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	41		23
Equity Securities [Abstract]
Equity securities			14
Commingled funds			9
Equity securities subtotal			23
Fixed income [Abstract]
Debt securities issued by the U.S. Treasury and other U.S. government corporations and agencies	148		130
Debt securities issued by states of the United States and political subdivisions of the states	29		37
Corporate debt securities	224		249
Federal agency mortgage-backed securities	29		49
Commercial mortgage-backed securities (non-agency)			6
Commingled funds fixed income			0
Fixed income subtotal	430		471
Direct lending securities	104		89
Other debt obligations			1
Pledged assets for Zion Station decommissioning subtotal	575	[9]	607	[9]
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Net assets (liabilities) excluded from nuclear decommissioning trust fund investments	5		7
Exelon Generation Co L L C [Member] | Rabbi Trust Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	0		1
Fixed income [Abstract]
Mutual funds	12	[10],[11]	13	[10],[11]
Rabbi trust investments subtotal	12		14
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash surrender value of life insurance investments excluded from Rabbi Trust investments	9		8
Exelon Generation Co L L C [Member] | Derivative Financial Instruments Assets [Member]
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Fair value swap contract current asset	85		226
Fair value swap contract noncurrent asset	0		0
Exelon Generation Co L L C [Member] | Commodity Mark To Market Derivative Assets [Member]
Fixed income [Abstract]
Other derivatives	3,975		4,901
Proprietary trading	2,577		3,193
Effect of netting and allocation of collateral received/(paid)	(5,152)	[6]	(6,056)	[6]
Mark-to-market subtotal	1,400	[12]	2,038	[12]
Exelon Generation Co L L C [Member] | Commodity Mark To Market Derivative Liabilities [Member]
Fixed income [Abstract]
Cash flow hedges	0
Other derivatives	(3,223)		(3,499)
Proprietary trading	(2,499)		(3,121)
Effect of netting and allocation of collateral received/(paid)	5,385	[6]	6,087	[6]
Mark-to-market subtotal	(337)		(533)
Exelon Generation Co L L C [Member] | Interest Rate Mark To Market Derivative Assets [Member]
Fixed income [Abstract]
Effect of netting and allocation of collateral received/(paid)	43		51
Interest rate mark to market	88		101
Interest rate mark-to-market Subtotal	45		50
Exelon Generation Co L L C [Member] | Interest Rate Mark To Market Derivative Liabilities [Member]
Fixed income [Abstract]
Effect of netting and allocation of collateral received/(paid)			(51)
Interest rate mark to market	(71)		(84)
Interest rate mark-to-market Subtotal	(28)		(33)
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	56		487	[1]
Fixed income [Abstract]
Other investments	2		2
Deferred compensation	0
Total assets	3,249		3,497
Total liabilities	(69)		(83)
Total net assets	3,180		3,414
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Deferred compensation	0
Collateral received from counterparties, net of collateral paid to counterparties	169				219
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 1 [Member] | Nuclear Decommissioning Trust Fund Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	216		245
Equity Securities [Abstract]
Equity securities	1,649		1,480
Commingled funds	0
Equity securities subtotal	1,649		1,480
Fixed income [Abstract]
Debt securities issued by the U.S. Treasury and other U.S. government corporations and agencies	1,039		1,057
Debt securities issued by states of the United States and political subdivisions of the states	0
Fixed income subtotal	1,039		1,057
Other debt obligations	0
Nuclear decommissioning trust fund investments subtotal	2,904	[2]	2,782	[2]
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 1 [Member] | Pledged Assets For Zion Station Decommissioning [Member]
Equity Securities [Abstract]
Equity securities			14
Equity securities subtotal			14
Fixed income [Abstract]
Debt securities issued by the U.S. Treasury and other U.S. government corporations and agencies	138		118
Fixed income subtotal	138		118
Pledged assets for Zion Station decommissioning subtotal	138	[9]	132	[9]
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 1 [Member] | Rabbi Trust Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	0		1
Fixed income [Abstract]
Mutual funds	12	[10],[11]	13	[10],[11]
Rabbi trust investments subtotal	12		14
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 1 [Member] | Commodity Mark To Market Derivative Assets [Member]
Fixed income [Abstract]
Other derivatives	801		861
Proprietary trading	764		1,042
Effect of netting and allocation of collateral received/(paid)	(1,428)	[6]	(1,823)	[6]
Mark-to-market subtotal	137	[12]	80	[12]
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 1 [Member] | Commodity Mark To Market Derivative Liabilities [Member]
Fixed income [Abstract]
Cash flow hedges	0
Other derivatives	(884)		(1,041)
Proprietary trading	(782)		(1,084)
Effect of netting and allocation of collateral received/(paid)	1,597	[6]	2,042	[6]
Mark-to-market subtotal	(69)		(83)
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 1 [Member] | Interest Rate Mark To Market Derivative Liabilities [Member]
Fixed income [Abstract]
Interest rate mark to market	0
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	0		0	[1]
Fixed income [Abstract]
Deferred compensation	(26)		(28)
Total assets	5,822		5,765
Total liabilities	(499)		(289)
Total net assets	5,323		5,476
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Deferred compensation	26		28
Collateral received from counterparties, net of collateral paid to counterparties	64				(155)
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 2 [Member] | Nuclear Decommissioning Trust Fund Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	0
Equity Securities [Abstract]
Equity securities	0
Commingled funds	2,073		1,933
Equity securities subtotal	2,073		1,933
Fixed income [Abstract]
Debt securities issued by the U.S. Treasury and other U.S. government corporations and agencies	0
Debt securities issued by states of the United States and political subdivisions of the states	328		321
Debt securities issued by foreign governments	108		93
Corporate debt securities	1,796		1,788
Federal agency mortgage-backed securities	23		24
Commercial mortgage-backed securities (non-agency)	43		45
Residential mortgage-backed securities (non-agency)	11		11
Mutual funds fixed income	26		23
Fixed income subtotal	2,335		2,305
Other debt obligations	16		15
Nuclear decommissioning trust fund investments subtotal	4,424	[2]	4,253	[2]
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 2 [Member] | Pledged Assets For Zion Station Decommissioning [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	41		23
Equity Securities [Abstract]
Commingled funds			9
Equity securities subtotal			9
Fixed income [Abstract]
Debt securities issued by the U.S. Treasury and other U.S. government corporations and agencies	10		12
Debt securities issued by states of the United States and political subdivisions of the states	29		37
Corporate debt securities	224		249
Federal agency mortgage-backed securities	29		49
Commercial mortgage-backed securities (non-agency)			6
Commingled funds fixed income			0
Fixed income subtotal	292		353
Direct lending securities	0
Other debt obligations			1
Pledged assets for Zion Station decommissioning subtotal	333	[9]	386	[9]
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 2 [Member] | Rabbi Trust Investments [Member]
Fixed income [Abstract]
Rabbi trust investments subtotal			0
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 2 [Member] | Commodity Mark To Market Derivative Assets [Member]
Fixed income [Abstract]
Other derivatives	2,535		3,173
Proprietary trading	1,637		2,078
Effect of netting and allocation of collateral received/(paid)	(3,152)	[6]	(4,175)	[6]
Mark-to-market subtotal	1,020	[12]	1,076	[12]
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 2 [Member] | Commodity Mark To Market Derivative Liabilities [Member]
Fixed income [Abstract]
Cash flow hedges	0
Other derivatives	(2,131)		(2,289)
Proprietary trading	(1,530)		(1,959)
Effect of netting and allocation of collateral received/(paid)	3,216	[6]	4,020	[6]
Mark-to-market subtotal	(445)		(228)
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 2 [Member] | Interest Rate Mark To Market Derivative Assets [Member]
Fixed income [Abstract]
Effect of netting and allocation of collateral received/(paid)	43		51
Interest rate mark to market	88		101
Interest rate mark-to-market Subtotal	45		50
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 2 [Member] | Interest Rate Mark To Market Derivative Liabilities [Member]
Fixed income [Abstract]
Effect of netting and allocation of collateral received/(paid)	(43)		(51)
Interest rate mark to market	(71)		(84)
Interest rate mark-to-market Subtotal	(28)		(33)
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	0		0	[1]
Equity Securities [Abstract]
Equity securities	0
Fixed income [Abstract]
Other investments	9		17
Deferred compensation	0
Total assets	566		1,171
Total liabilities	177		(222)
Total net assets	743		949
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Deferred compensation	0
Collateral received from counterparties, net of collateral paid to counterparties	0				(33)
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 3 [Member] | Nuclear Decommissioning Trust Fund Investments [Member]
Equity Securities [Abstract]
Commingled funds	0
Equity securities subtotal	0
Fixed income [Abstract]
Debt securities issued by the U.S. Treasury and other U.S. government corporations and agencies	0
Debt securities issued by states of the United States and political subdivisions of the states	0
Mutual funds fixed income	0
Fixed income subtotal	0
Direct lending securities	210		183
Other debt obligations	0
Nuclear decommissioning trust fund investments subtotal	210	[2]	183	[2]
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 3 [Member] | Pledged Assets For Zion Station Decommissioning [Member]
Fixed income [Abstract]
Direct lending securities	104		89
Pledged assets for Zion Station decommissioning subtotal	104	[9]	89	[9]
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 3 [Member] | Rabbi Trust Investments [Member]
Fixed income [Abstract]
Rabbi trust investments subtotal			0
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 3 [Member] | Commodity Mark To Market Derivative Assets [Member]
Fixed income [Abstract]
Other derivatives	639		867
Proprietary trading	176		73
Effect of netting and allocation of collateral received/(paid)	(572)	[6]	(58)	[6]
Mark-to-market subtotal	243	[12]	882	[12]
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 3 [Member] | Commodity Mark To Market Derivative Liabilities [Member]
Fixed income [Abstract]
Cash flow hedges	0
Other derivatives	(208)		(169)
Proprietary trading	(187)		(78)
Effect of netting and allocation of collateral received/(paid)	572	[6]	25	[6]
Mark-to-market subtotal	177		(222)
Exelon Generation Co L L C [Member] | Fair Value Inputs Level 3 [Member] | Interest Rate Mark To Market Derivative Liabilities [Member]
Fixed income [Abstract]
Interest rate mark to market	0
Commonwealth Edison Co [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	0		111	[13]
Fixed income [Abstract]
Deferred compensation	(9)		(8)
Total assets	7		119
Total liabilities	(169)		(301)
Total net assets	(162)		(182)
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Deferred compensation	9		8
Mark-to-market derivative liabilities (current liabilities)	15		18
Mark-to-market derivative liabilities (noncurrent liabilities)	60		49
Commonwealth Edison Co [Member] | Rabbi Trust Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	0
Fixed income [Abstract]
Mutual funds	7		8
Rabbi trust investments subtotal	7		8
Commonwealth Edison Co [Member] | Derivative Financial Instruments Assets [Member]
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Fair value swap contract current asset	85		226
Fair value swap contract noncurrent asset	0		0
Commonwealth Edison Co [Member] | Derivative Financial Instruments Liabilities [Member]
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Fair value of energy swap contract current liability	15		18
Fair value of energy swap contract noncurrent liability	60		49
Commonwealth Edison Co [Member] | Commodity Mark To Market Derivative Liabilities [Member]
Fixed income [Abstract]
Mark-to-market subtotal	(160)	[14],[8]	(293)	[14],[8]
Commonwealth Edison Co [Member] | Fair Value Inputs Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	0		111	[13]
Fixed income [Abstract]
Deferred compensation	0
Total assets	7		119
Total liabilities	0
Total net assets	7		119
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Deferred compensation	0
Commonwealth Edison Co [Member] | Fair Value Inputs Level 1 [Member] | Rabbi Trust Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	0
Fixed income [Abstract]
Mutual funds	7		8
Rabbi trust investments subtotal	7		8
Commonwealth Edison Co [Member] | Fair Value Inputs Level 1 [Member] | Derivative Financial Instruments Liabilities [Member]
Fixed income [Abstract]
Mark-to-market subtotal	0	[14],[8]
Commonwealth Edison Co [Member] | Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	0
Fixed income [Abstract]
Deferred compensation	(9)		(8)
Total assets	0
Total liabilities	(9)		(8)
Total net assets	(9)		(8)
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Deferred compensation	9		8
Commonwealth Edison Co [Member] | Fair Value Inputs Level 2 [Member] | Rabbi Trust Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	0
Fixed income [Abstract]
Mutual funds	0
Rabbi trust investments subtotal	0
Commonwealth Edison Co [Member] | Fair Value Inputs Level 2 [Member] | Derivative Financial Instruments Liabilities [Member]
Fixed income [Abstract]
Mark-to-market subtotal	0	[14],[8]
Commonwealth Edison Co [Member] | Fair Value Inputs Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	0
Fixed income [Abstract]
Deferred compensation	0
Total assets	0
Total liabilities	(160)		(293)
Total net assets	(160)		(293)
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Deferred compensation	0
Commonwealth Edison Co [Member] | Fair Value Inputs Level 3 [Member] | Rabbi Trust Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	0
Fixed income [Abstract]
Mutual funds	0
Rabbi trust investments subtotal	0
Commonwealth Edison Co [Member] | Fair Value Inputs Level 3 [Member] | Commodity Mark To Market Derivative Liabilities [Member]
Fixed income [Abstract]
Mark-to-market subtotal	(160)	[14],[8]	(293)	[14],[8]
PECO Energy Co [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	225		346
Fixed income [Abstract]
Deferred compensation	(18)		(18)
Total assets	234		355
Total liabilities	(18)		(18)
Total net assets	216		337
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Deferred compensation	18		18
PECO Energy Co [Member] | Rabbi Trust Investments [Member]
Fixed income [Abstract]
Mutual funds	9		9
Rabbi trust investments subtotal	9	[15],[16]	9	[15],[16]
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash surrender value of life insurance investments excluded from Rabbi Trust investments	14		13
PECO Energy Co [Member] | Fair Value Inputs Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	225		346
Fixed income [Abstract]
Total assets	234		355
Total net assets	234		355
PECO Energy Co [Member] | Fair Value Inputs Level 1 [Member] | Rabbi Trust Investments [Member]
Fixed income [Abstract]
Mutual funds	9		9
Rabbi trust investments subtotal	9	[15],[16]	9	[15],[16]
PECO Energy Co [Member] | Fair Value Inputs Level 2 [Member]
Fixed income [Abstract]
Deferred compensation	(18)		(18)
Total liabilities	(18)		(18)
Total net assets	(18)		(18)
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Deferred compensation	18		18
PECO Energy Co [Member] | Fair Value Inputs Level 2 [Member] | Rabbi Trust Investments [Member]
Fixed income [Abstract]
Mutual funds			0
PECO Energy Co [Member] | Fair Value Inputs Level 3 [Member] | Rabbi Trust Investments [Member]
Fixed income [Abstract]
Mutual funds			0
Baltimore Gas and Electric Company [Member] | Rabbi Trust Investments [Member]
Fixed income [Abstract]
Mutual funds	5	[15]	5	[15]
Baltimore Gas and Electric Company [Member] | Derivative Financial Instruments Assets [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	58		33
Fixed income [Abstract]
Total assets	63		38
Total net assets	58		33
Baltimore Gas and Electric Company [Member] | Derivative Financial Instruments Liabilities [Member]
Fixed income [Abstract]
Deferred compensation	(5)		(5)
Total liabilities	(5)		(5)
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Deferred compensation	5		5
Baltimore Gas and Electric Company [Member] | Fair Value Inputs Level 1 [Member] | Rabbi Trust Investments [Member]
Fixed income [Abstract]
Mutual funds	5	[15]	5	[15]
Baltimore Gas and Electric Company [Member] | Fair Value Inputs Level 1 [Member] | Derivative Financial Instruments Assets [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Cash equivalents	58		33
Fixed income [Abstract]
Total assets	63		38
Total net assets	63		38
Baltimore Gas and Electric Company [Member] | Fair Value Inputs Level 2 [Member] | Derivative Financial Instruments Assets [Member]
Fixed income [Abstract]
Total net assets	(5)		(5)
Baltimore Gas and Electric Company [Member] | Fair Value Inputs Level 2 [Member] | Derivative Financial Instruments Liabilities [Member]
Fixed income [Abstract]
Deferred compensation	(5)		(5)
Total liabilities	(5)		(5)
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis [Abstract]
Deferred compensation	$ 5		$ 5

[1] Excludes certain cash equivalents considered to be held-to-maturity and not reported at fair value.

[2] Excludes net assets of $21 million and $30 million at March 31, 2013 and December 31, 2012, respectively. These items consist of receivables related to pending securities sales, interest and dividend receivables, and payables related to pending securities purchases.

[3] Excludes net assets of $5 million and $7 million at March 31, 2013 and December 31, 2012, respectively. These items consist of receivables related to pending securities sales, interest and dividend receivables, and payables related to pending securities purchases.

[4] The mutual funds held by the Rabbi trusts include $51 million related to deferred compensation and $15 million related to Supplemental Executive Retirement Plan at March 31, 2013, and $53 million related to deferred compensation and $16 million related to Supplemental Executive Retirement Plan at December 31, 2012. These funds are classified as Level 1 as they are valued based upon quoted prices (unadjusted) in active markets.

[5] Excludes $29 million and $28 million of the cash surrender value of life insurance investments at March 31, 2013 and December 31, 2012, respectively.

[6] Includes collateral postings (received) from counterparties. Collateral (received) from counterparties, net of collateral paid to counterparties, totaled $169 million, $64 million and $0 million allocated to Level 1, Level 2 and Level 3 mark-to-market derivatives, respectively, as of March 31, 2013. Collateral (received) from counterparties, net of collateral paid to counterparties, totaled $219 million, $(155) million and $(33) million allocated to Level 1, Level 2 and Level 3 mark-to-market derivatives, respectively, as of December 31, 2012.

[7] The Level 3 balance does not include current and noncurrent assets for Generation and current and noncurrent liabilities for ComEd of $85 million and $0 million at March 31, 2013 and $226 million and $0 million at December 31, 2012, respectively, related to the fair value of Generation's financial swap contract with ComEd.

[8] The Level 3 balance includes the current and noncurrent liability of $15 million and $60 million at March 31, 2013, respectively, and $18 million and $49 million at December 31, 2012, respectively, related to floating-to-fixed energy swap contracts with unaffiliated suppliers.

[9] Excludes net assets of $5 million and $7 million at March 31, 2013 December 31, 2012, respectively. These items consist of receivables related to pending securities sales, interest and dividend receivables, and payables related to pending securities purchases.

[10] The $12 million mutual funds held by the Rabbi trusts are classified as Level 1 as they are valued based upon quoted prices (unadjusted) in active markets.

[11] Excludes $9 million and $8 million of the cash surrender value of life insurance investments at March 31, 2013 and December 31, 2012, respectively.

[12] The Level 3 balance includes current and noncurrent assets for Generation of $85 million and $0 million at March 31, 2013 and $226 million and $0 million at December 31, 2012, respectively, related to the fair value of Generation's financial swap contract with ComEd, which eliminates upon consolidation in Exelon's Consolidated Financial Statements.

[13] The mutual funds held by the Rabbi trusts are classified as Level 1 as they are valued based upon quoted prices (unadjusted) in active markets.

[14] The Level 3 balance includes the current liability of $85 million at March 31, 2013, and $226 million at December 31, 2012, related to the fair value of ComEd's financial swap contract with Generation which eliminates upon consolidation in Exelon's Consolidated Financial Statements.

[15] Excludes $14 million and $13 million of the cash surrender value of life insurance investments at March 31, 2013 and December 31, 2012, respectively.

[16]