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Retirement Benefits Retirement Benefits- Narrative (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |
Increase (Decrease) in Obligation, Pension Benefits |  | $ 23 |  |  |  |  |  |  |
Benefit obligation increase (decrease) reflecting actual census data |  | (14) |  |  |  |  |  |  |
Accumulated Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), after Tax |  | 18 |  |  |  |  |  |  |
Regulatory asset increase (decrease) due to updated valuation adjustment |  | 61 |  |  |  |  |  |  |
Defined benefit plan, amounts recognized In regulatory liabilities, before tax |  | $ 1 |  |  |  |  |  |  |
Defined Contribution Plan, Employer Discretionary Contribution Amount | [1] |  |  | $ 179 |  | $ 128 |  | $ 164 |
Fitzpatrick [Member] |  |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |
Increase (Decrease) in Obligation, Pension Benefits |  |  |  | 16 |  |  |  |  |
Benefit obligation increase (decrease) reflecting actual census data |  |  |  | $ (17) |  |  |  |  |
Projected Benefit Obligation In Excess Of Plan Assets [Member] |  |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |
Defined Benefit Plan, Funded Percentage |  |  |  | 81.00% |  | 80.00% |  |  |
Accumulated Benefit Obligation In Excess Of Plan Assets [Member] |  |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |
Defined Benefit Plan, Funded Percentage |  |  |  | 85.00% |  | 88.00% |  |  |
Pension Plan, Defined Benefit [Member] |  |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |
Regulatory asset increase (decrease) due to updated valuation adjustment |  |  |  | $ (6) |  | $ (423) | [2] | $ 959 |
Expected return on plan assets |  |  |  | 7.00% |  | 7.00% |  | 7.00% |
Discount rate | [4] |  |  | 3.62% | [3],[5] | 4.04% | [6],[7] | 4.29% | [8]
Defined Contribution Plan, Employer Discretionary Contribution Amount |  |  |  | $ 300 |  |  |  |  |
Other Postretirement Benefit Plan, Defined Benefit [Member] |  |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |
Regulatory asset increase (decrease) due to updated valuation adjustment |  |  |  | $ (57) |  | $ 125 | [2] | $ 95 |
Expected return on plan assets |  |  |  | 6.60% |  | 6.58% |  | 6.71% |
Discount rate | [4] |  |  | 3.61% | [3],[5] | 4.04% | [6],[7] | 4.29% | [7],[8]
Change in Assumptions for Defined Benefit Plans [Member] | Pension Plan, Defined Benefit [Member] |  |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |
Expected return on plan assets |  |  |  | 7.00% |  |  |  |  |
Discount rate |  |  |  | 3.62% |  |  |  |  |
Change in Assumptions for Defined Benefit Plans [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member] |  |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |
Expected return on plan assets |  |  |  | 6.60% |  |  |  |  |
Discount rate |  |  |  | 3.61% |  |  |  |  |
Assets [Member] | Pension Plan, Defined Benefit [Member] |  |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |
Discount rate |  |  |  | (4.86%) |  |  |  |  |
Assets [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member] |  |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |
Discount rate |  |  |  | (4.66%) |  |  |  |  |
Subsequent Event [Member] | Pension Plan, Defined Benefit [Member] |  |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |
Expected return on plan assets |  |  | 7.00% |  |  |  |  |  |
Subsequent Event [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member] |  |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |
Expected return on plan assets |  |  | 6.67% |  |  |  |  |  |
[1] | Includes $13 million related to CENG for the year ended December 31, 2016
[2] | 2016 amounts include PHI for the period of March 24, 2016 through December 31, 2016.
[3] | (a)The discount rates above represent the blended rates used to establish the majority of Exelon’s pension and other postretirement benefits costs for the year ended December 31, 2018. Certain benefit plans used individual rates ranging from 3.49%-3.65% and 3.57%-3.68% for pension and other postretirement plans, respectively.
[4] | (d)Not applicable to pension and other postretirement benefit plans that do not have plan assets.
[5] | (e)3.25% through 2019 and 3.75% thereafter.
[6] | (b)The discount rates above represent the blended rates used to establish the majority of Exelon's pension and other postretirement benefits costs for the year ended December 31, 2017. Certain benefit plans used individual rates ranging from 3.66%-4.11% and 4.00%-4.17% for pension and other postretirement plans, respectively.
[7] | (f)The legacy Exelon, CEG and CENG pension and other postretirement plans used a rate of compensation increase of 3.25% through 2019 and 3.75% thereafter, while the legacy PHI pension and other postretirement plans used a weighted-average rate of compensation increase of 5% for all periods.
[8] | (c)The discount rates above represent the blended rates used to establish the majority of Exelon’s pension and other postretirement benefits costs for the year ended December 31, 2016. Certain benefit plans used the individual rates ranging from 3.68%-4.14% and 4.32%-4.43% for pension and other postretirement plans, respectively.

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