Income Taxes - Tax Effects of Temporary Differences and Carryforwards (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Operating Loss Carryforwards [Line Items]
Plant basis differences	$ (12,143)	$ (11,612)
Accrual based contracts	(178)	(214)
Derivatives and other financial instruments	(46)	(509)
Deferred pension and postretirement obligation	1,914	1,489
Nuclear decommissioning activities	(726)	(647)
Deferred debt refinancing costs	112	173
Regulatory assets and liabilities	(1,824)	(1,611)
Tax loss carryforward	111	252
Tax credit carryforward	97	534
Investment in CENG	(563)	(541)
Other, net	1,029	804
Deferred income tax liabilities (net)	12,217	11,882
Unamortized investment tax credits	(555)	(490)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(12,772)	(12,372)
Exelon Generation Co L L C [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(3,834)	(3,879)
Accrual based contracts	(178)	(214)
Derivatives and other financial instruments	(79)	(505)
Deferred pension and postretirement obligation	(390)	(362)
Nuclear decommissioning activities	(726)	(646)
Deferred debt refinancing costs	57	79
Regulatory assets and liabilities	0	0
Tax loss carryforward	48	76
Tax credit carryforward	143	534
Investment in CENG	(563)	(541)
Other, net	346	67
Deferred income tax liabilities (net)	5,176	5,391
Unamortized investment tax credits	(528)	(454)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(5,704)	(5,845)
Commonwealth Edison Co [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(3,945)	(3,523)
Accrual based contracts	0	0
Derivatives and other financial instruments	(4)	(4)
Deferred pension and postretirement obligation	(543)	(522)
Nuclear decommissioning activities	0	0
Deferred debt refinancing costs	(18)	(21)
Regulatory assets and liabilities	(286)	(241)
Tax loss carryforward	0	47
Tax credit carryforward	0	0
Investment in CENG	0	0
Other, net	255	154
Deferred income tax liabilities (net)	4,541	4,110
Unamortized investment tax credits	(20)	(22)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(4,561)	(4,132)
PECO Energy Co [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(2,749)	(2,573)
Accrual based contracts	0	0
Derivatives and other financial instruments	0	0
Deferred pension and postretirement obligation	2	0
Nuclear decommissioning activities	0	0
Deferred debt refinancing costs	(2)	(3)
Regulatory assets and liabilities	27	42
Tax loss carryforward	11	11
Tax credit carryforward	0	0
Investment in CENG	0	0
Other, net	111	122
Deferred income tax liabilities (net)	2,600	2,401
Unamortized investment tax credits	(2)	(3)
Total deferred income tax liabilities (net) and unamortized investment tax credits	(2,602)	(2,404)
Baltimore Gas and Electric Company [Member]
Operating Loss Carryforwards [Line Items]
Plant basis differences	(1,661)	(1,538)
Accrual based contracts	0	0
Derivatives and other financial instruments	0	0
Deferred pension and postretirement obligation	(53)	(74)
Nuclear decommissioning activities	0	0
Deferred debt refinancing costs	(4)	(5)
Regulatory assets and liabilities	(258)	(253)
Tax loss carryforward	39	52
Tax credit carryforward	0	0
Investment in CENG	0	0
Other, net	30	26
Deferred income tax liabilities (net)	1,907	1,792
Unamortized investment tax credits	(5)	(6)
Total deferred income tax liabilities (net) and unamortized investment tax credits	$ (1,912)	$ (1,798)