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, 2017		Dec. 31, 2016
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	[1]	$ 16,791		$ 14,347
Defined Benefit Plan, Fair Value of Plan Assets		18,573		16,791	[1]
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		208		167
Relating to assets still held at the reporting date		11		(2)
Defined Benefit Plan Purchases		31		69
Defined Benefit Plan Sales		16		14
Settlements	[2]	0		(12)
Defined Benefit Plan, Fair Value of Plan Assets		234		208
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		206		165
Relating to assets still held at the reporting date		11		(2)
Defined Benefit Plan Purchases		31		69
Defined Benefit Plan Sales		16		14
Settlements	[2]	0		(12)
Defined Benefit Plan, Fair Value of Plan Assets		232		206
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 Purchases		0		0
Defined Benefit Plan Sales		0		0
Settlements	[2]	0		0
Defined Benefit Plan, Fair Value of Plan Assets		2		2
Fair Value, Measurements, Recurring [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Alternative Investment, Fair Value Disclosure		3,534		3,113
Fair value of net plan assets at beginning of year	[3],[4],[5]	19,397
Defined Benefit Plan, Fair Value of Plan Assets	[4],[5]	21,303	[6]	19,397	[3]
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],[5]	208
Defined Benefit Plan, Fair Value of Plan Assets	[4],[5]	234	[6]	208	[3]
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	[5]	8,440	[6]	7,897	[3]
Fair value of net plan assets at beginning of year	[3],[5]	16,819
Defined Benefit Plan, Fair Value of Plan Assets	[5]	18,575	[6]	16,819	[3]
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	[5]	756	[6]	748	[3]
Fair value of net plan assets at beginning of year	[3],[5]	6,199
Defined Benefit Plan, Fair Value of Plan Assets	[5]	6,739	[6]	6,199	[3]
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	[3],[5]	208
Defined Benefit Plan, Fair Value of Plan Assets	[5]	234	[6]	208	[3]
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	[3],[5]	206
Defined Benefit Plan, Fair Value of Plan Assets	[5]	$ 232	[6]	$ 206	[3]

[1] 2016 amounts include PHI for the period of March 24, 2016 through December 31, 2016.

[2] Represents cash settlements only.

[3] Effective March 23, 2016, Exelon became sponsor of PHI's defined benefit pension and other postretirement benefit plans, and assumed PHI's benefit plan obligations and related assets.

[4] Excludes net assets of $2 million and net liabilities of $28 million at December 31, 2017 and 2016, respectively, which are required to reconcile to the fair value of net plan assets. These items consist primarily of receivables related to pending securities sales, interest and dividends receivable, and payables related to pending securities purchases.

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

[6] 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.