Fair Value of Financial Assets and Liabilities - Fair Value Reconciliation of Level 3 Assets and Liabilities Measured on a Recurring Basis (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Ending balance
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	749		656
Included in net income	532		(42)
Included in other comprehensive income			2
Included in noncurrent payables to affiliates			0
Included in payable for Zion Station decommissioning	(2)		0
Included in regulatory assets/liabilities	0	[1]	(118)	[1]
Change in collateral	198		7
Purchases	598		297
Sales	(80)		(86)
Settlements	(64)		(18)
Transfers into Level 3 - (Asset) / Liability	7		(87)
Transfers out of Level 3 - (Asset) / Liability	(207)		(36)
Ending balance	1,721		749
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities	644		168
Fair Value, Inputs, Level 3 [Member] | Nuclear Decommissioning Trust Fund Investment Level Three Asset [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	350
Included in net income	6
Included in payable for Zion Station decommissioning	0
Change in collateral	0
Purchases	400		203
Sales	(15)		(28)
Settlements	(64)		(18)
Transfers into Level 3 - (Asset) / Liability	0		0
Transfers out of Level 3 - (Asset) / Liability	0		0
Ending balance	691		350
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities	4		1
Fair Value, Inputs, Level 3 [Member] | Pledged Assets For Zion Station Decommissioning [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	112
Purchases			62
Sales	(50)		(39)
Settlements	0		0
Transfers into Level 3 - (Asset) / Liability	0		0
Transfers out of Level 3 - (Asset) / Liability	0		0
Ending balance	184		112
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities	0		0
Fair Value, Inputs, Level 3 [Member] | Derivative [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	465
Purchases	76	[2]	28
Sales	(7)		(11)
Settlements	0		0
Transfers into Level 3 - (Asset) / Liability	7		(86)	[1]
Transfers out of Level 3 - (Asset) / Liability	(201)		(35)
Ending balance	1,050		465
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities	640		156
Fair Value, Inputs, Level 3 [Member] | Other Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	15
Included in noncurrent payables to affiliates			(8)
Included in regulatory assets/liabilities			(218)
Purchases	2		4
Sales	(8)		(8)
Settlements	0		0
Transfers into Level 3 - (Asset) / Liability	0		(1)
Transfers out of Level 3 - (Asset) / Liability	(6)		(1)
Ending balance	3		15
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities	0		0
Exelon Generation Co L L C [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Settlements			(215)
Gain (loss) reclassified to results of operating due to the settlement of derivative contracts	114		(207)
Increase (Decrease) in Fair Value of Interest Rate Fair Value Hedging Instruments			11
Exelon Generation Co L L C [Member] | Interest Rate Swap [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Settlements	(1)		7
Exelon Generation Co L L C [Member] | Derivative [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Cash	34
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	942		949
Included in net income	532		(49)
Included in other comprehensive income			(217)
Included in noncurrent payables to affiliates			8
Included in payable for Zion Station decommissioning	(2)		0
Included in regulatory assets/liabilities			0	[1]
Change in collateral	198		7
Purchases	598		297
Sales	(80)		(86)
Settlements	(64)		(18)
Transfers into Level 3 - (Asset) / Liability	7		(87)
Transfers out of Level 3 - (Asset) / Liability	(207)		(36)
Ending balance	1,928		942
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities	644		157
Decrease in Fair Value Adjustment	13		133
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Nuclear Decommissioning Trust Fund Investment Level Three Asset [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance			183
Included in net income			2
Included in other comprehensive income			0
Included in noncurrent payables to affiliates	14
Included in payable for Zion Station decommissioning			0
Change in collateral			0
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Pledged Assets For Zion Station Decommissioning [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance			89
Included in net income	0		0
Included in other comprehensive income			0
Included in noncurrent payables to affiliates			0
Included in payable for Zion Station decommissioning	(2)		0
Included in regulatory assets/liabilities	0	[1]	0	[1]
Change in collateral	0		0
Purchases	120
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance			660
Included in net income	526		(51)	[3],[4]
Included in other comprehensive income			(219)	[3]
Included in noncurrent payables to affiliates			0
Included in payable for Zion Station decommissioning	0		0
Included in regulatory assets/liabilities	0	[1]	0	[1]
Change in collateral	198		7
Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Other Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance			17
Included in net income	0		0
Included in other comprehensive income			2
Included in noncurrent payables to affiliates			0
Included in payable for Zion Station decommissioning	0		0
Included in regulatory assets/liabilities	0	[1]	0	[1]
Change in collateral	0		0
Commonwealth Edison Co [Member] | Fair Value, Inputs, Level 3 [Member] | Other Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(193)	[3],[5]	(293)	[3],[5]
Included in net income	0	[3]	0	[3],[5]
Included in other comprehensive income			0	[3],[5]
Included in noncurrent payables to affiliates			0	[3],[5]
Included in payable for Zion Station decommissioning	0	[3]	0	[3],[5]
Included in regulatory assets/liabilities	(14)	[1],[3]	100	[1],[3],[5]
Change in collateral	0	[3]	0	[3],[5]
Purchases	0	[3]	0	[3],[5]
Sales	0	[3]	0	[3],[5]
Settlements	0	[3]	0	[3],[5]
Transfers into Level 3 - (Asset) / Liability	0	[3]	0	[3],[5]
Transfers out of Level 3 - (Asset) / Liability	0	[3]	0	[3],[5]
Ending balance	(207)	[3]	(193)	[3],[5]
The amount of total gains (losses) included in income attributed to the change in unrealized gains (losses) related to assets and liabilities	0	[3]	0	[3],[5]
Consolidation, Eliminations [Member] | Fair Value, Inputs, Level 3 [Member] | Nuclear Decommissioning Trust Fund Investment Level Three Asset [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Purchases			0
Sales			0
Settlements			0
Transfers into Level 3 - (Asset) / Liability			0
Transfers out of Level 3 - (Asset) / Liability			0
Consolidation, Eliminations [Member] | Exelon Generation Co L L C [Member] | Fair Value, Inputs, Level 3 [Member] | Nuclear Decommissioning Trust Fund Investment Level Three Asset [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Included in net income			7
Included in other comprehensive income			219
Included in noncurrent payables to affiliates	(14)
Included in payable for Zion Station decommissioning			0
Included in regulatory assets/liabilities	14
Change in collateral			$ 0

[1] Includes an increase of transfers into Level 3 arising from reductions in market liquidity, which resulted in less observable contract tenures in various locations.

[2] Includes $34 million of fair value from contracts acquired as a result of the Integrys acquisition.

[3] Includes $13 million and $133 million of decreases in fair value and $1 million and $(7) million of realized gains (losses) due to settlements associated with floating-to-fixed energy swap contracts with unaffiliated suppliers for the years ended December 31, 2014 and 2013, respectively.

[4] Includes the reclassification of $114 million and $207 million of realized gains due to the settlement of derivative contracts for the years ended December 31, 2014 and 2013, respectively.

[5] Includes $11 million of decreases in fair value and realized gains due to settlements of $215 million associated with Generation's financial swap contract with ComEd for the year ended December 31, 2013. All items eliminate upon consolidation in Exelon’s Consolidated Financial Statements.