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Pension and Other Postretirement Benefit Plans - Narrative (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Accumulated benefit obligation |  | $ 20,500 | $ 18,400 |
Percentage of Company's pension investments categorized as Level 3 assets (as percent) |  | 2.00% | 2.00% |
Employer contributions to defined contribution savings plans |  | $ 131 | $ 126 | $ 125
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) |  | 13.00% |  |
United States | Minimum |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Expected rate of return on plan assets (as percent) |  | 8.00% |  |
United States | Maximum |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Expected rate of return on plan assets (as percent) |  | 8.75% |  |
Forecast | United States | Minimum |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Expected rate of return on plan assets (as percent) | 7.70% |  |  |
Forecast | United States | Maximum |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Expected rate of return on plan assets (as percent) | 8.30% |  |  |
U.S. Equity Securities | United States | Minimum |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage |  | 35.00% |  |
U.S. Equity Securities | United States | Maximum |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage |  | 55.00% |  |
International Securities | United States | Minimum |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage |  | 20.00% |  |
International Securities | United States | Maximum |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage |  | 35.00% |  |
Fixed Income Investments | United States | Minimum |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage |  | 20.00% |  |
Fixed Income Investments | United States | Maximum |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage |  | 35.00% |  |
Cash And Other Investments | United States | Maximum |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage |  | 5.00% |  |
Other Postretirement Benefits |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Assets no longer restricted to the payment of postretirement benefits |  | $ 0 | 992 |
Fair value of plan assets |  | 1,114 | 1,019 | 1,913
Expected contributions to the pension plans and other postretirement benefit plans during next fiscal year |  | 25 |  |
Estimated net loss (gain) amounts that will be amortized from AOCI into net pension and postretirement benefit cost during next fiscal year |  | 1 |  |
Estimated prior service cost (credit) amounts that will be amortized from AOCI into net pension and postretirement benefit cost during next fiscal year |  | (84) |  |
Pension Plan |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Estimated net loss (gain) amounts that will be amortized from AOCI into net pension and postretirement benefit cost during next fiscal year |  | 314 |  |
Estimated prior service cost (credit) amounts that will be amortized from AOCI into net pension and postretirement benefit cost during next fiscal year |  | (64) |  |
Pension Plan | Significant Unobservable Inputs (Level 3) |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets |  | 488 | 435 |
International | Pension Plan |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Assets no longer restricted to the payment of postretirement benefits |  | 0 | 0 |
Fair value of plan assets |  | 9,339 | $ 7,794 | $ 7,204
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% | 5.60% | 5.70%
United States |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Expected rate of return on plan assets (as percent) |  | 8.70% | 8.60% | 8.50%
United States | Pension Plan |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Assets no longer restricted to the payment of postretirement benefits |  | $ 0 | $ 0 |
Accumulated benefit obligation |  | 11,500 | 10,500 |
Fair value of plan assets |  | 10,896 | $ 9,766 | $ 9,266
Expected contributions to the pension plans and other postretirement benefit plans during next fiscal year |  | 60 |  |
Estimated net loss (gain) amounts that will be amortized from AOCI into net pension and postretirement benefit cost during next fiscal year |  | 230 |  |
Estimated prior service cost (credit) amounts that will be amortized from AOCI into net pension and postretirement benefit cost during next fiscal year |  | $ (51) |  |

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- Definition 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. + References No definition available. + Details
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- Definition Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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