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Pension and Other Postretirement Benefit Plans - Narrative (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Accumulated benefit obligation |  | $ 19,000 | $ 20,500 |
Employer contributions to defined contribution savings plans |  | $ 136 | $ 131 | $ 126
Pension Benefits |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Percentage of Company's pension investments categorized as Level 3 assets (as percent) |  | 5.00% | 2.00% |
Estimated net loss (gain) amounts that will be amortized from AOCI into net pension and postretirement benefit cost during next fiscal year |  | $ 204 |  |
Estimated prior service cost (credit) amounts that will be amortized from AOCI into net pension and postretirement benefit cost during next fiscal year |  | (62) |  |
Pension Benefits | Significant Unobservable Inputs (Level 3) |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 826 | $ 488 |
Other Postretirement Benefits |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 968 | 1,114 | $ 1,019
Expected contributions to the pension plans and other postretirement benefit plans during next fiscal year |  | 15 |  |
Estimated net loss (gain) amounts that will be amortized from AOCI into net pension and postretirement benefit cost during next fiscal year |  | (7) |  |
Estimated prior service cost (credit) amounts that will be amortized from AOCI into net pension and postretirement benefit cost during next fiscal year |  | (78) |  |
Other Postretirement Benefits | Significant Unobservable Inputs (Level 3) |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | $ 0 | $ 0 |
United States |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Expected annual standard deviation in returns of the target portfolio which reflects both the equity allocation and the diversification benefits among the asset classes in which the portfolio invests (as percent) |  | 11.00% |  |
Expected rate of return on plan assets (as percent) |  | 8.20% | 8.70% | 8.60%
United States | Minimum |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Expected rate of return on plan assets (as percent) |  | 7.70% |  |
United States | Minimum | Forecast |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Expected rate of return on plan assets (as percent) | 7.70% |  |  |
United States | Maximum |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Expected rate of return on plan assets (as percent) |  | 8.30% |  |
United States | Maximum | Forecast |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Expected rate of return on plan assets (as percent) | 8.10% |  |  |
United States | U.S. equities | Minimum |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Target allocation (as percent) |  | 30.00% |  |
United States | U.S. equities | Maximum |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Target allocation (as percent) |  | 50.00% |  |
United States | International equities | Minimum |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Target allocation (as percent) |  | 15.00% |  |
United States | International equities | Maximum |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Target allocation (as percent) |  | 30.00% |  |
United States | Fixed Income Investments | Minimum |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Target allocation (as percent) |  | 30.00% |  |
United States | Fixed Income Investments | Maximum |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Target allocation (as percent) |  | 45.00% |  |
United States | Cash And Other Investments | Maximum |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Target allocation (as percent) |  | 5.00% |  |
United States | Pension Benefits |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Accumulated benefit obligation |  | $ 10,400 | $ 11,500 |
Fair value of plan assets |  | 9,648 | 10,896 | $ 9,766
Expected contributions to the pension plans and other postretirement benefit plans during next fiscal year |  | 50 |  |
Estimated net loss (gain) amounts that will be amortized from AOCI into net pension and postretirement benefit cost during next fiscal year |  | 141 |  |
Estimated prior service cost (credit) amounts that will be amortized from AOCI into net pension and postretirement benefit cost during next fiscal year |  | (50) |  |
United States | Pension Benefits | Significant Unobservable Inputs (Level 3) |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 13 | 15 | 18
International | Pension Benefits |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 8,580 | $ 9,339 | $ 7,794
Expected contributions to the pension plans and other postretirement benefit plans during next fiscal year |  | $ 150 |  |
Expected rate of return on plan assets (as percent) |  | 5.10% | 5.10% | 5.60%
International | Pension Benefits | Significant Unobservable Inputs (Level 3) |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | $ 813 | $ 473 | $ 417

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