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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
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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

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