| **Postretirement Benefits - Effect of Significant Inputs (Details) - USD ($) $ in Millions**              | 12 Months Ended |
|-----------------------------------------------------------------------------------------------------------|-----------------|
||
| Health and Life Insurance Benefits                                                                        |            |   |
| **Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]**                              |            |   |
| Fair value of plan assets at beginning of year                                                            | $ 415           | $ 447 |
| Fair value of plan assets at end of year                                                                  | 369             | 415 |
| Health and Life Insurance Benefits | Hedge Funds [Member]                                                 |            |   |
| **Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]**                              |            |   |
| Fair value of plan assets at beginning of year                                                            | 22              |   |
| Fair value of plan assets at end of year                                                                  | 22              | 22 |
| Health and Life Insurance Benefits | Hedge Funds [Member] | Fair Value, Inputs, Level 3 [Member]          |            |   |
| **Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]**                              |            |   |
| Fair value of plan assets at beginning of year                                                            | 22              | 21 |
| Unrealized gains                                                                                          |            | 1 |
| Realized gains                                                                                            | 0               | 0 |
| Purchases, issuances, and settlements                                                                     | 0               | 0 |
| Fair value of plan assets at end of year                                                                  | 22              | 22 |
| Health and Life Insurance Benefits | Private Equity Funds [Member]                                        |            |   |
| **Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]**                              |            |   |
| Fair value of plan assets at beginning of year                                                            | 23              |   |
| Fair value of plan assets at end of year                                                                  | 20              | 23 |
| Health and Life Insurance Benefits | Private Equity Funds [Member] | Fair Value, Inputs, Level 3 [Member] |            |   |
| **Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]**                              |            |   |
| Fair value of plan assets at beginning of year                                                            | 23              | 26 |
| Unrealized gains                                                                                          |            | 3 |
| Realized gains                                                                                            | 1               | 4 |
| Purchases, issuances, and settlements                                                                     | (4)             | (10) |
| Fair value of plan assets at end of year                                                                  | 20              | 23 |
| Pension Benefits                                                                                          |            |   |
| **Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]**                              |            |   |
| Fair value of plan assets at beginning of year                                                            | 2,627           | 2,519 |
| Fair value of plan assets at end of year                                                                  | 2,422           | 2,627 |
| Pension Benefits | Hedge Funds [Member]                                                                   |            |   |
| **Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]**                              |            |   |
| Fair value of plan assets at beginning of year                                                            | 106             |   |
| Fair value of plan assets at end of year                                                                  | 109             | 106 |
| Pension Benefits | Hedge Funds [Member] | Fair Value, Inputs, Level 3 [Member]                            |            |   |
| **Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]**                              |            |   |
| Fair value of plan assets at beginning of year                                                            | 106             | 101 |
| Unrealized gains                                                                                          | 2               | 5 |
| Realized gains                                                                                            | 1               | 0 |
| Purchases, issuances, and settlements                                                                     | 0               | 0 |
| Fair value of plan assets at end of year                                                                  | 109             | 106 |
| Pension Benefits | Private Equity Funds [Member]                                                          |            |   |
| **Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]**                              |            |   |
| Fair value of plan assets at beginning of year                                                            | 94              |   |
| Fair value of plan assets at end of year                                                                  | 79              | 94 |
| Pension Benefits | Private Equity Funds [Member] | Fair Value, Inputs, Level 3 [Member]                   |            |   |
| **Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]**                              |            |   |
| Fair value of plan assets at beginning of year                                                            | 94              | 103 |
| Unrealized gains                                                                                          | 0               | 10 |
| Realized gains                                                                                            | 5               | 15 |
| Purchases, issuances, and settlements                                                                     | (20)            | (34) |
| Fair value of plan assets at end of year                                                                  | 79              | 94 |
| Pension Benefits | Real Estate [Member]                                                                   |            |   |
| **Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]**                              |            |   |
| Fair value of plan assets at beginning of year                                                            | 1               |   |
| Fair value of plan assets at end of year                                                                  | 1               | 1 |
| Pension Benefits | Real Estate [Member] | Fair Value, Inputs, Level 3 [Member]                            |            |   |
| **Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]**                              |            |   |
| Fair value of plan assets at beginning of year                                                            | 1               | 1 |
| Unrealized gains                                                                                          | 0               | 0 |
| Realized gains                                                                                            | 0               | 0 |
| Purchases, issuances, and settlements                                                                     | 0               | 0 |
| Fair value of plan assets at end of year                                                                  | 1               | 1 |
| Pension Benefits | Fixed Income Interest Rate [Member]                                                    |            |   |
| **Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]**                              |            |   |
| Fair value of plan assets at beginning of year                                                            | 1               |   |
| Fair value of plan assets at end of year                                                                  | 0               | 1 |
| Pension Benefits | Fixed Income Interest Rate [Member] | Fair Value, Inputs, Level 3 [Member]             |            |   |
| **Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]**                              |            |   |
| Fair value of plan assets at beginning of year                                                            | 1               | (13) |
| Unrealized gains                                                                                          | (1)             | 14 |
| Realized gains                                                                                            | 8               | 0 |
| Purchases, issuances, and settlements                                                                     | (8)             | 0 |
| Fair value of plan assets at end of year                                                                  | 0               | 1 |
| Pension Benefits | Equity [Member]                                                                        |            |   |
| **Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]**                              |            |   |
| Fair value of plan assets at beginning of year                                                            | (106)           |   |
| Fair value of plan assets at end of year                                                                  | 0               | (106) |
| Pension Benefits | Equity [Member] | Fair Value, Inputs, Level 3 [Member]                                 |            |   |
| **Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]**                              |            |   |
| Fair value of plan assets at beginning of year                                                            | (106)           | (72) |
| Unrealized gains                                                                                          | 138             | (43) |
| Realized gains                                                                                            | (165)           | 0 |
| Purchases, issuances, and settlements                                                                     | 133             | 9 |
| Fair value of plan assets at end of year                                                                  | $ 0             | $ (106) |

| X                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Defined Benefit Plans, Realized Gains
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Actual return on plan assets, separately identifying the amount related to assets sold during the period.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Actual return on plan assets, separately identifying the amount related to assets still held at the reporting date.
+ References
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| - Definition
A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.
+ References
No definition available.
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of increase (decrease) in plan assets from purchases, sales and settlement of plan obligations.
+ References
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