Basis of Presentation - Narrative (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Total interest expense to affiliates, net	$ 10	$ 10
Pretax decrease in operating and maintenance expense	(2,081)	(1,858)
Pretax increase in taxes other than income	304	293
Income (Loss) from Equity Method Investments		(19)
Income (Loss) from Equity Method Investments, Net of Dividends or Distributions	1
Exelon Generation Co L L C [Member]
Purchased power and fuel from affiliates	7	349
Total interest expense to affiliates, net	12	12
Pretax decrease in operating and maintenance expense	(1,162)	(938)
Pretax increase in taxes other than income	122	105
Income (Loss) from Equity Method Investments		(19)
Baltimore Gas and Electric Company [Member]
Total interest expense to affiliates, net	4	4
Purchased power and fuel	350	409
Pretax decrease in operating and maintenance expense	(156)	(163)
Pretax increase in taxes other than income	57	60
Income (Loss) from Equity Method Investments		$ 0