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Pension and Other Postretirement Benefit Plans - Fair Values of Pension Plan Assets (Details) - Pension Plan - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 488	$ 435
United States
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10,896	9,766	$ 9,266
Fair Value of Plan Assets Excluding Net Asset Value Investments	5,189	4,542
Investments measured at NAV practical expedient	5,707	5,224
United States | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6	4
United States | Developed markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	390	521
United States | Emerging markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	138	104
United States | Developed markets, Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,743	2,521
United States | Government and agency obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	757	475
United States | Corporate obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	900	660
United States | Mortgage and asset backed-securities, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	240	239
United States | Other, Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15	18
United States | Quoted Prices In Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets Excluding Net Asset Value Investments	3,277	3,148
United States | Quoted Prices In Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6	2
United States | Quoted Prices In Active Markets for Identical Assets (Level 1) | Developed markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	390	521
United States | Quoted Prices In Active Markets for Identical Assets (Level 1) | Emerging markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	138	104
United States | Quoted Prices In Active Markets for Identical Assets (Level 1) | Developed markets, Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,743	2,521
United States | Quoted Prices In Active Markets for Identical Assets (Level 1) | Government and agency obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Quoted Prices In Active Markets for Identical Assets (Level 1) | Corporate obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Quoted Prices In Active Markets for Identical Assets (Level 1) | Mortgage and asset backed-securities, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Quoted Prices In Active Markets for Identical Assets (Level 1) | Other, Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets Excluding Net Asset Value Investments	1,897	1,376
United States | Significant Other Observable Inputs (Level 2) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	2
United States | Significant Other Observable Inputs (Level 2) | Developed markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Significant Other Observable Inputs (Level 2) | Emerging markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Significant Other Observable Inputs (Level 2) | Developed markets, Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Significant Other Observable Inputs (Level 2) | Government and agency obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	757	475
United States | Significant Other Observable Inputs (Level 2) | Corporate obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	900	660
United States | Significant Other Observable Inputs (Level 2) | Mortgage and asset backed-securities, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	240	239
United States | Significant Other Observable Inputs (Level 2) | Other, Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets Excluding Net Asset Value Investments	15	18	23
United States | Significant Unobservable Inputs (Level 3) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Significant Unobservable Inputs (Level 3) | Developed markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Significant Unobservable Inputs (Level 3) | Emerging markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Significant Unobservable Inputs (Level 3) | Real estate, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets Excluding Net Asset Value Investments	0	0	0
United States | Significant Unobservable Inputs (Level 3) | Developed markets, Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Significant Unobservable Inputs (Level 3) | Government and agency obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Significant Unobservable Inputs (Level 3) | Corporate obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Significant Unobservable Inputs (Level 3) | Mortgage and asset backed-securities, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Significant Unobservable Inputs (Level 3) | Insurance contracts, Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets Excluding Net Asset Value Investments	0	0	0
United States | Significant Unobservable Inputs (Level 3) | Other, Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15	18
Fair Value of Plan Assets Excluding Net Asset Value Investments	15	18	23
International
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9,339	7,794	7,204
Fair Value of Plan Assets Excluding Net Asset Value Investments	8,537	7,006
Investments measured at NAV practical expedient	802	788
International | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	73	53
International | Developed markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,888	3,033
International | Emerging markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	238	172
International | Government and agency obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,344	2,027
International | Corporate obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	334	284
International | Fixed income obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11	9
International | Real estate, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	7
International | Developed markets, Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	660	565
International | Government and agency obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	268	237
International | Corporate obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	119	92
International | Mortgage and asset backed-securities, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	55	50
International | Insurance contracts, Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	537	471
International | Other, Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7	6
International | Quoted Prices In Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets Excluding Net Asset Value Investments	1,602	952
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	54	42
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Developed markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	562	187
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Emerging markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	62	24
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Government and agency obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	249	123
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Corporate obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	2
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Fixed income obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7	6
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Real estate, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Developed markets, Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	660	565
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Government and agency obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2	2
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Corporate obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	0
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Mortgage and asset backed-securities, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Insurance contracts, Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
International | Quoted Prices In Active Markets for Identical Assets (Level 1) | Other, Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1
International | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets Excluding Net Asset Value Investments	6,462	5,637
International | Significant Other Observable Inputs (Level 2) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	19	11
International | Significant Other Observable Inputs (Level 2) | Developed markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,326	2,846
International | Significant Other Observable Inputs (Level 2) | Emerging markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	176	148
International | Significant Other Observable Inputs (Level 2) | Government and agency obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,095	1,904
International | Significant Other Observable Inputs (Level 2) | Corporate obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	329	282
International | Significant Other Observable Inputs (Level 2) | Fixed income obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	3
International | Significant Other Observable Inputs (Level 2) | Real estate, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	3
International | Significant Other Observable Inputs (Level 2) | Developed markets, Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
International | Significant Other Observable Inputs (Level 2) | Government and agency obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	266	235
International | Significant Other Observable Inputs (Level 2) | Corporate obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	118	92
International | Significant Other Observable Inputs (Level 2) | Mortgage and asset backed-securities, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	55	50
International | Significant Other Observable Inputs (Level 2) | Insurance contracts, Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	67	59
International | Significant Other Observable Inputs (Level 2) | Other, Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6	4
International | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets Excluding Net Asset Value Investments	473	417	400
International | Significant Unobservable Inputs (Level 3) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
International | Significant Unobservable Inputs (Level 3) | Developed markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
International | Significant Unobservable Inputs (Level 3) | Emerging markets equities, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
International | Significant Unobservable Inputs (Level 3) | Government and agency obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
International | Significant Unobservable Inputs (Level 3) | Corporate obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
International | Significant Unobservable Inputs (Level 3) | Fixed income obligations, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
International | Significant Unobservable Inputs (Level 3) | Real estate, Investment funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2	4
Fair Value of Plan Assets Excluding Net Asset Value Investments	2	4	5
International | Significant Unobservable Inputs (Level 3) | Developed markets, Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
International | Significant Unobservable Inputs (Level 3) | Government and agency obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
International | Significant Unobservable Inputs (Level 3) | Corporate obligations, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
International | Significant Unobservable Inputs (Level 3) | Mortgage and asset backed-securities, Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
International | Significant Unobservable Inputs (Level 3) | Insurance contracts, Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	470	412
Fair Value of Plan Assets Excluding Net Asset Value Investments	470	412	393
International | Significant Unobservable Inputs (Level 3) | Other, Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	1
Fair Value of Plan Assets Excluding Net Asset Value Investments	$ 1	$ 1	$ 2

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- Definition

Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

+ 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 (b)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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