Retirement Benefits Retirement Benefits - Reconciliation of Level 3 Assets and Liabilities measured at Fair Value for Pension and Other Postretirement Benefit Plans (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017
Pension Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of net plan assets at beginning of year		$ 18,573		$ 16,791
Defined Benefit Plan, Fair Value of Plan Assets		16,678		18,573
Pension Plan, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of net plan assets at beginning of year		458		437
Relating to assets still held at the reporting date		(5)		40
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Sold		(1)
Defined Benefit Plan Purchases		54		36
Defined Benefit Plan Sales		8		16
Settlements		(12)	[1]	(39)
Defined Benefit Plan, Fair Value of Plan Assets		486		458
Pension Plan, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of net plan assets at beginning of year		232		206
Relating to assets still held at the reporting date		(14)		11
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Sold		(1)
Defined Benefit Plan Purchases		19		31
Defined Benefit Plan Sales		8		16
Settlements		(12)	[1]	0
Defined Benefit Plan, Fair Value of Plan Assets		216		232
Pension Plan, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member] | Equity Security Individually Held [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of net plan assets at beginning of year		2		2
Relating to assets still held at the reporting date		0		0
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Sold		0
Defined Benefit Plan Purchases		0		0
Defined Benefit Plan Sales		0		0
Settlements		0	[1]	0
Defined Benefit Plan, Fair Value of Plan Assets		2		2
Pension Plan, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member] | Private Placement [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of net plan assets at beginning of year		224		229	[2]
Relating to assets still held at the reporting date		9		29
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Sold		0
Defined Benefit Plan Purchases		35		5
Defined Benefit Plan Sales		0		0
Settlements		0	[1]	(39)
Defined Benefit Plan, Fair Value of Plan Assets		268		224
Fair Value, Measurements, Recurring [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment, Fair Value Disclosure		3,433		3,534
Fair value of net plan assets at beginning of year	[3],[4]	21,303
Defined Benefit Plan, Fair Value of Plan Assets	[3],[4]	19,130	[5]	21,303
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of net plan assets at beginning of year	[3],[4]	458
Defined Benefit Plan, Fair Value of Plan Assets	[3],[4]	486	[5]	458
Fair Value, Measurements, Recurring [Member] | Pension Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment, Fair Value Disclosure	[4]	7,247	[5]	8,216
Fair value of net plan assets at beginning of year	[4]	18,575
Defined Benefit Plan, Fair Value of Plan Assets	[4]	16,720	[5]	18,575
Fair Value, Measurements, Recurring [Member] | Pension Plan, Defined Benefit [Member] | Debt Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment, Fair Value Disclosure	[4]	613	[5]	756
Fair value of net plan assets at beginning of year	[4]	6,739
Defined Benefit Plan, Fair Value of Plan Assets	[4]	6,102	[5]	6,739
Fair Value, Measurements, Recurring [Member] | Pension Plan, Defined Benefit [Member] | Private Placement [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment, Fair Value Disclosure	[4]	798	[5]	695
Fair value of net plan assets at beginning of year	[4]	919
Defined Benefit Plan, Fair Value of Plan Assets	[4]	1,066	[5]	919
Fair Value, Measurements, Recurring [Member] | Pension Plan, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of net plan assets at beginning of year	[4]	458
Defined Benefit Plan, Fair Value of Plan Assets	[4]	486	[5]	458
Fair Value, Measurements, Recurring [Member] | Pension Plan, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member] | Debt Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of net plan assets at beginning of year	[4]	232
Defined Benefit Plan, Fair Value of Plan Assets	[4]	216	[5]	232
Fair Value, Measurements, Recurring [Member] | Pension Plan, Defined Benefit [Member] | Fair Value, Inputs, Level 3 [Member] | Private Placement [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of net plan assets at beginning of year	[4]	224
Defined Benefit Plan, Fair Value of Plan Assets	[4]	$ 268	[5]	$ 224

[1] (b)Represents cash settlements only.

[2] (a)Prior year amounts reflect a reclassification from Not subject to leveling into Level 3.

[3] Excludes net liabilities of $44 million and net assets of $2 million at December 31, 2018 and 2017, respectively, which are required to reconcile to the fair value of net plan assets. These items consist primarily of receivables or payables related to pending securities sales and purchases, interest and dividends receivable

[4] See Note 11—Fair Value of Financial Assets and Liabilities for a description of levels within the fair value hierarchy

[5] Effective March 31, 2017, Exelon became sponsor of FitzPatrick's defined benefit pension and other postretirement benefit plans, and assumed FitzPatrick's benefit plan obligations