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Pension and Other Postretirement Benefit Plans - Summary of Changes in Fair Value of Company's Level 3 Pension Plan Assets (Details) - Pension Benefits - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019
Level 3 |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |
Fair value of plan assets January 1 | $ 860 |
Actual return on plan assets: |  |
Fair value of plan assets December 31 | 942 | $ 860
United States |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |
Fair value of plan assets January 1 | 11,361 | 9,648
Actual return on plan assets: |  |
Fair value of plan assets December 31 | 12,672 | 11,361
United States | Other |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |
Fair value of plan assets January 1 | 9 |
Actual return on plan assets: |  |
Fair value of plan assets December 31 | 7 | 9
United States | Level 3 |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |
Fair value of plan assets January 1 | 9 | 13
Actual return on plan assets: |  |
Relating to assets still held at December 31 | (5) | (8)
Relating to assets sold during the year | 5 | 8
Purchases and sales, net | (2) | (4)
Fair value of plan assets December 31 | 7 | 9
United States | Level 3 | Insurance Contracts |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |
Fair value of plan assets January 1 | 0 | 0
Actual return on plan assets: |  |
Relating to assets still held at December 31 | 0 | 0
Relating to assets sold during the year | 0 | 0
Purchases and sales, net | 0 | 0
Fair value of plan assets December 31 | 0 | 0
United States | Level 3 | Real Estate |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |
Fair value of plan assets January 1 | 0 | 0
Actual return on plan assets: |  |
Relating to assets still held at December 31 | 0 | 0
Relating to assets sold during the year | 0 | 0
Purchases and sales, net | 0 | 0
Fair value of plan assets December 31 | 0 | 0
United States | Level 3 | Other |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |
Fair value of plan assets January 1 | 9 | 13
Actual return on plan assets: |  |
Relating to assets still held at December 31 | (5) | (8)
Relating to assets sold during the year | 5 | 8
Purchases and sales, net | (2) | (4)
Fair value of plan assets December 31 | 7 | 9
International |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |
Fair value of plan assets January 1 | 10,163 | 8,580
Actual return on plan assets: |  |
Fair value of plan assets December 31 | 12,049 | 10,163
International | Insurance Contracts |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |
Fair value of plan assets January 1 | 916 |
Actual return on plan assets: |  |
Fair value of plan assets December 31 | 1,012 | 916
International | Real Estate |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |
Fair value of plan assets January 1 | 1 |
Actual return on plan assets: |  |
Fair value of plan assets December 31 | 16 | 1
International | Other |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |
Fair value of plan assets January 1 | 21 |
Actual return on plan assets: |  |
Fair value of plan assets December 31 | 6 | 21
International | Level 3 |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |
Fair value of plan assets January 1 | 851 | 813
Actual return on plan assets: |  |
Relating to assets still held at December 31 | 103 | 54
Purchases and sales, net | (17) | (16)
Transfers out of Level 3 | (2) | 0
Fair value of plan assets December 31 | 935 | 851
International | Level 3 | Insurance Contracts |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |
Fair value of plan assets January 1 | 851 | 811
Actual return on plan assets: |  |
Relating to assets still held at December 31 | 103 | 54
Purchases and sales, net | (17) | (14)
Transfers out of Level 3 | (2) | 0
Fair value of plan assets December 31 | 935 | 851
International | Level 3 | Real Estate |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |
Fair value of plan assets January 1 | 0 | 1
Actual return on plan assets: |  |
Relating to assets still held at December 31 | 0 | 0
Purchases and sales, net | 0 | (1)
Transfers out of Level 3 | 0 | 0
Fair value of plan assets December 31 | 0 | 0
International | Level 3 | Other |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |
Fair value of plan assets January 1 | 0 | 1
Actual return on plan assets: |  |
Relating to assets still held at December 31 | 0 | 0
Purchases and sales, net | 0 | (1)
Transfers out of Level 3 | 0 | 0
Fair value of plan assets December 31 | $ 0 | $ 0

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- Definition Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets sold. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(02)(A) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921 + Details
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