Derivative Financial Instruments - Narrative (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2014 MWh		Dec. 31, 2013 MWh		Dec. 31, 2012 MWh
Derivative [Line Items]
Gain Loss On Interest Rate Swap Treasury Rate Lock Net Of Tax		$ 20		$ 5
Estimated percentage of natural gas purchases hedged		30.00%
Derivative, notional amount		1,491		2,651
Pre-tax net income impact associated with a hypothetical 10% increase in interest rates - exclusive upper bound		8
Ineffective portion recognized in income		18		2
Notional amount of interest rate swaps acquired from merger		1,900
Mark-to-market derivative liabilities		403		300
Mark-to-market derivative liabilities		234		159
Fair value of interest rate swaps from merger acquiree		150
Cash collateral held		69
Letters of credit held		16
Net Asset Liability Position After Contractual Right Of Offset		(90)
Credit Derivative, Collateral Held Directly or by Third Parties Monetary Amount		2
PHI Merger [Member]
Derivative [Line Items]
Mark-to-market derivative liabilities		100
Interest Rate Swap [Member]
Derivative [Line Items]
Mark-to-market derivative liabilities		28
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional amount of preissuance interest rate cash flow hedge derivatives		3,070
Reclassification out of Accumulated Other Comprehensive Income [Member] | Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Notional amount of preissuance interest rate cash flow hedge derivatives		26
Reclassification out of Accumulated Other Comprehensive Income [Member] | Energy Related Hedges [Member] | Operating Revenue One [Member]
Derivative [Line Items]
Cash flow hedge activity impact on pre-tax net income based on reclassification adjustment from accumulated other comprehensive income		195		464		747
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member] | Fair Value Hedging [Member]
Derivative [Line Items]
Derivative, notional amount		1,450		1,275
Increase in notional amount of derivative instruments		100
Increase in notional amount of derivative instruments		75
Derivative asset		29		26
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Notional amount of preissuance interest rate cash flow hedge derivatives		400
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member] | ExGen Texas Power [Member]
Derivative [Line Items]
Derivative, notional amount		505
Interest Rate Swap [Member] | Derivative [Member]
Derivative [Line Items]
Derivative, notional amount		1,450
Interest Rate Contract [Member] | Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Notional amount of preissuance interest rate cash flow hedge derivatives		400
Interest Rate Contract [Member] | Designated as Hedging Instrument [Member] | ExGen Texas Power [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Mark-to-market derivative liabilities		8
Baltimore Gas and Electric Company [Member]
Derivative [Line Items]
Net receivable from electric utility		40
Credit exposure under off system sales		8
Incremental collateral for loss of investment grade credit rating		79
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Proprietary trading activities volume		10,571,000		8,762,000		12,958,000
Hypothetical increase in interest rates associated with variable-rate debt		50.00%
Unrealized gain (loss) on interest rate cash flow hedges, pretax, accumulated other comprehensive income (loss)	21
Mark-to-market derivative liabilities		105		120
Mark-to-market derivative liabilities		214		142
Derivative, collateral, right to reclaim cash		8		10
Expected reclassification from accumulated other comprehensive income to results of operations		2
Letters of credit posted		672		364
Cash collateral held		77		206
Letters of credit held		24		34
Cash collateral posted				72
Incremental collateral for loss of investment grade credit rating		2,400		2,000
Net Asset Liability Position After Contractual Right Of Offset		16
Exelon Generation Co L L C [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional amount of preissuance interest rate cash flow hedge derivatives		770
Derivative asset		6,813		3,960
Mark-to-market derivative liabilities		1,540		804
Mark-to-market derivative liabilities		4,947		2,023
Exelon Generation Co L L C [Member] | Designated as Hedging Instrument [Member] | Antelope Valle [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
DOE interest rate swap		485
Exelon Generation Co L L C [Member] | Designated as Hedging Instrument [Member] | Other Solar Projects [Member]
Derivative [Line Items]
Notional amounts on forward starting interest rate swaps		26
Exelon Generation Co L L C [Member] | Total Cash Flow Hedges [Member]
Derivative [Line Items]
Cash flow hedge activity impact on pre-tax net income based on reclassification adjustment from accumulated other comprehensive income		195		683		1,368
Exelon Generation Co L L C [Member] | Total Cash Flow Hedges [Member] | Operating Revenue One [Member]
Derivative [Line Items]
Net unrealized pre-tax gain (loss) on effective cash flow hedges						5
Exelon Generation Co L L C [Member] | Derivative [Member]
Derivative [Line Items]
Derivative, collateral, right to reclaim cash		1,497
Exelon Generation Co L L C [Member] | Derivative [Member] | Other Solar Projects [Member]
Derivative [Line Items]
Mark-to-market derivative liabilities		3
Exelon Generation Co L L C [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Mark-to-market derivative liabilities		2
Exelon Generation Co L L C [Member] | Interest Rate Swap [Member] | Designated as Hedging Instrument [Member] | Fair Value Hedging [Member]
Derivative [Line Items]
Derivative, notional amount				550
Interest rate swaps previously held by acquiree		550
Derivative asset		7		23
Exelon Generation Co L L C [Member] | Interest Rate Swap [Member] | Derivative [Member]
Derivative [Line Items]
Derivative, notional amount		550
Exelon Generation Co L L C [Member] | Interest Rate Contract [Member]
Derivative [Line Items]
Derivative, notional amount		126
Exelon Generation Co L L C [Member] | Interest Rate Contract [Member] | Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Increase in notional amount of derivative instruments		213
Notional amount of interest rate swaps acquired from merger		26
Exelon Generation Co L L C [Member] | Foreign Exchange Contract [Member]
Derivative [Line Items]
Derivative, notional amount		349
Commonwealth Edison Co [Member]
Derivative [Line Items]
Term of contract		20 years
Mark-to-market derivative liabilities		187		176
Mark-to-market derivative liabilities		20		17
Net receivable from electric utility		43
Cash collateral held		19
Commonwealth Edison Co [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative asset		0	[1]	0
Mark-to-market derivative liabilities		187	[1]	176	[1]
Mark-to-market derivative liabilities		20	[1]	17	[1]
PECO Energy Co [Member]
Derivative [Line Items]
Mark-to-market derivative liabilities		14
Net receivable from electric utility		29
Credit exposure under natural gas supply and management agreements		8
Incremental collateral for loss of investment grade credit rating		$ 36
Minimum [Member]
Derivative [Line Items]
Expected generation hedged in next twelve months		93.00%
Expected generation hedged in year two		61.00%
Expected generation hedged in year three		31.00%
Minimum [Member] | Baltimore Gas and Electric Company [Member]
Derivative [Line Items]
Estimated percentage of natural gas purchases hedged		10.00%
Maximum [Member]
Derivative [Line Items]
Expected generation hedged in next twelve months		96.00%
Expected generation hedged in year two		64.00%
Expected generation hedged in year three		34.00%
Maximum [Member] | Baltimore Gas and Electric Company [Member]
Derivative [Line Items]
Estimated percentage of natural gas purchases hedged		20.00%

[1] Includes current and noncurrent liabilities relating to floating-to-fixed energy swap contracts with unaffiliated suppliers.