Derivative Financial Instruments - Summary of Credit Risk Exposure (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Cash collateral held		$ 8
Letters of credit held		19
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Cash collateral held		35	$ 24
Letters of credit held		33	$ 28
Total Exposure Before Credit Collateral [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Investment grade		738
Non-investment grade		90
Internally rated — investment grade		253
Internally rated — non-investment grade		83
Total		1,164
Credit Collateral [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Investment grade		4
Non-investment grade		12
Internally rated — investment grade		0
Internally rated — non-investment grade		11
Total	[1]	27
Net Exposure [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Investment grade		734
Non-investment grade		78
Internally rated — investment grade		253
Internally rated — non-investment grade		72
Total		1,137
Number Of Counterparties Greater Than Ten Percent Of Net Exposure [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Investment grade		1
Non-investment grade		0
Internally rated — investment grade		0
Internally rated — non-investment grade		0
Total		1
Net Exposure Of Counterparties Greater Than Ten Percent Of Net Exposure [Member] | Exelon Generation Co L L C [Member]
Derivative [Line Items]
Investment grade		244
Non-investment grade		0
Internally rated — investment grade		0
Internally rated — non-investment grade		0
Total		$ 244

[1] As of December 31, 2017, credit collateral held from counterparties where Generation had credit exposure included $8 million of cash and $19 million of letters of credit. The credit collateral does not include non-liquid collateral.