Fair Value of Financial Assets and Liabilities - Fair Value Assets Liabilities Measured On Recurring Basis Unobservable Input Reconciliation (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Exelon Generation Co L L C [Member]
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Gain (loss) reclassified to results of operating due to the settlement of derivative contracts	$ 87	$ 83	$ 20	$ 156
Exelon Generation Co L L C [Member] | Nuclear Decommissioning Trust Fund Investment Level Three Asset [Member]
Purchases, sales, issuances and settlements
Settlements				(11)
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Abstract]
Beginning balance	843	793	749	656
Total realized / unrealized gains (losses)
Included in net income	77	(32)	(279)	(6)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)				(219)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Included In Noncurrent Payables To Affiliates	3	(1)	14	234
Included in payable for Zion Station decommissioning	2	0	(2)	(1)
Included in noncurrent payables to affiliates	(44)	(37)	15	(55)
Change in collateral	(79)	30	(257)	(13)
Purchases, sales, issuances and settlements
Purchases	148	41	455	151
Sales	(33)	(29)	(66)	(70)
Settlements	(27)	(3)	(46)	(11)
Transfers into Level 3	(21)	(4)	9	(11)
Transfers out of Level 3	1	(5)	(26)	(4)
Ending balance	1,066	701	1,066	701
The amount of total losses included in income attributed to the change in unrealized gains (losses) related to assets and liabilities held	164	51	(261)	160
Fair Value, Inputs, Level 3 [Member] | Exelon Generation Co L L C [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Abstract]
Beginning balance	977	878	942	949
Total realized / unrealized gains (losses)
Included in net income	77	(32)	(279)	(6)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)				(219)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Included In Noncurrent Payables To Affiliates	3	(1)	14	8
Included in payable for Zion Station decommissioning	2	0	(2)	(1)
Included in noncurrent payables to affiliates	0	0	0	0
Change in collateral	(79)	30	(257)	(13)
Purchases, sales, issuances and settlements
Purchases	148	41	455	151
Sales	(33)	(29)	(66)	(70)
Settlements	(27)	(3)	(46)	(11)
Transfers into Level 3	(21)	(4)	9	(11)
Transfers out of Level 3	1	(5)	(26)	(4)
Ending balance	1,244	823	1,244	823
The amount of total losses included in income attributed to the change in unrealized gains (losses) related to assets and liabilities held	164	51	(261)	149
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Decrease in Fair Value Adjustment				11
Fair Value, Inputs, Level 3 [Member] | Exelon Generation Co L L C [Member] | Nuclear Decommissioning Trust Fund Investment Level Three Asset [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Abstract]
Beginning balance	592	240	350	183
Total realized / unrealized gains (losses)
Included in net income	1	0	5	2
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)				0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Included In Noncurrent Payables To Affiliates	3	(1)	14	8
Included in payable for Zion Station decommissioning	0	0	0	0
Included in noncurrent payables to affiliates	0		0	0
Change in collateral	0	0	0	0
Purchases, sales, issuances and settlements
Purchases	83	23	331	90
Sales	(8)	(14)	(10)	(27)
Settlements	(27)	(3)	(46)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Ending balance	644	245	644	245
The amount of total losses included in income attributed to the change in unrealized gains (losses) related to assets and liabilities held	1	0	3	1
Fair Value, Inputs, Level 3 [Member] | Exelon Generation Co L L C [Member] | Pledged Assets For Zion Station Decommissioning [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Abstract]
Beginning balance	133	111	112	89
Total realized / unrealized gains (losses)
Included in net income	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)				0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Included In Noncurrent Payables To Affiliates	0	0	0	0
Included in payable for Zion Station decommissioning	2	0	(2)	(1)
Included in noncurrent payables to affiliates	0	0	0	0
Change in collateral	0	0	0	0
Purchases, sales, issuances and settlements
Purchases	53	10	95	43
Sales	(18)	(15)	(43)	(27)
Settlements	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Ending balance	166	106	166	106
The amount of total losses included in income attributed to the change in unrealized gains (losses) related to assets and liabilities held	0	0	0	0
Fair Value, Inputs, Level 3 [Member] | Exelon Generation Co L L C [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Abstract]
Beginning balance	242	516	465	660
Total realized / unrealized gains (losses)
Included in net income	76	(32)	(284)	(8)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)				(219)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Included In Noncurrent Payables To Affiliates	0	0	0	0
Included in payable for Zion Station decommissioning	0	0	0	0
Included in noncurrent payables to affiliates	0	0	0	0
Change in collateral	(79)	30	(257)	(13)
Purchases, sales, issuances and settlements
Purchases	12	8	27	16
Sales	0	0	(6)	(8)
Settlements	0	0	0	0
Transfers into Level 3	(21)	(4)	9	(11)
Transfers out of Level 3	1	(5)	(19)	(4)
Ending balance	431	461	431	461
The amount of total losses included in income attributed to the change in unrealized gains (losses) related to assets and liabilities held	163	51	(264)	148
Fair Value, Inputs, Level 3 [Member] | Exelon Generation Co L L C [Member] | Other Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Abstract]
Beginning balance	10	11	15	17
Total realized / unrealized gains (losses)
Included in net income	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)				0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Included In Noncurrent Payables To Affiliates	0	0	0	0
Included in payable for Zion Station decommissioning	0	0	0	0
Included in noncurrent payables to affiliates	0	0	0	0
Change in collateral	0	0	0	0
Purchases, sales, issuances and settlements
Purchases	0	0	2	2
Sales	(7)	0	(7)	(8)
Settlements	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	(7)	0
Ending balance	3	11	3	11
The amount of total losses included in income attributed to the change in unrealized gains (losses) related to assets and liabilities held	0	0	0	0
Fair Value, Inputs, Level 3 [Member] | Commonwealth Edison Co [Member]
Purchases, sales, issuances and settlements
Settlements	1	1	4	5
Footnotes To Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Decrease in Fair Value Adjustment			19
Increase In Fair Value Adjustment	45	37		57
Fair Value, Inputs, Level 3 [Member] | Commonwealth Edison Co [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Abstract]
Beginning balance	(134)	(85)	(193)	(293)
Total realized / unrealized gains (losses)
Included in net income	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)				0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Included In Noncurrent Payables To Affiliates	0	0	0	226
Included in payable for Zion Station decommissioning	0	0	0	0
Included in noncurrent payables to affiliates	(44)	(37)	15	(55)
Change in collateral	0	0	0	0
Purchases, sales, issuances and settlements
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Ending balance	(178)	(122)	(178)	(122)
The amount of total losses included in income attributed to the change in unrealized gains (losses) related to assets and liabilities held	0	0	0	11
Interest Rate Swap [Member] | Exelon Generation Co L L C [Member]
Purchases, sales, issuances and settlements
Settlements				$ 215