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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 Plan - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
United States |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |
Balance January 1 | $ 4,542 |
Actual return on plan assets: |  |
Balance December 31 | 5,189 | $ 4,542
United States | Significant Unobservable Inputs (Level 3) |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |
Balance January 1 | 18 | 23
Actual return on plan assets: |  |
Relating to assets still held at December 31 | (2) | (3)
Relating to assets sold during the year | 4 | 4
Purchases and sales, net | (5) | (6)
Balance December 31 | 15 | 18
United States | Significant Unobservable Inputs (Level 3) | Insurance Contracts |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |
Balance 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
Balance December 31 | 0 | 0
United States | Significant Unobservable Inputs (Level 3) | Real Estate |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |
Balance 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
Balance December 31 | 0 | 0
United States | Significant Unobservable Inputs (Level 3) | Other |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |
Balance January 1 | 18 | 23
Actual return on plan assets: |  |
Relating to assets still held at December 31 | (2) | (3)
Relating to assets sold during the year | 4 | 4
Purchases and sales, net | (5) | (6)
Balance December 31 | 15 | 18
International |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |
Balance January 1 | 7,006 |
Actual return on plan assets: |  |
Balance December 31 | 8,537 | 7,006
International | Significant Unobservable Inputs (Level 3) |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |
Balance January 1 | 417 | 400
Actual return on plan assets: |  |
Relating to assets still held at December 31 | 52 | (8)
Purchases and sales, net | 3 | (1)
Transfers into Level 3 | 1 | 26
Balance December 31 | 473 | 417
International | Significant Unobservable Inputs (Level 3) | Insurance Contracts |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |
Balance January 1 | 412 | 393
Actual return on plan assets: |  |
Relating to assets still held at December 31 | 52 | (9)
Purchases and sales, net | 5 | 2
Transfers into Level 3 | 1 | 26
Balance December 31 | 470 | 412
International | Significant Unobservable Inputs (Level 3) | Real Estate |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |
Balance January 1 | 4 | 5
Actual return on plan assets: |  |
Relating to assets still held at December 31 | 0 | 1
Purchases and sales, net | (2) | (2)
Transfers into Level 3 | 0 | 0
Balance December 31 | 2 | 4
International | Significant Unobservable Inputs (Level 3) | Other |  |
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] |  |
Balance January 1 | 1 | 2
Actual return on plan assets: |  |
Relating to assets still held at December 31 | 0 | 0
Purchases and sales, net | 0 | (1)
Transfers into Level 3 | 0 | 0
Balance December 31 | $ 1 | $ 1

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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/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5)(iv)(02)(A) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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