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Pension and Other Postretirement Plan Assets (Detail) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | ---
Pension Benefits |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Pension and other postretirement plan assets |  | $ 3,932 | $ 3,355 | $ 2,975
Assets measured at NAV | [1] | 2,211 | 1,843 |
Pension Benefits | Equity Securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Pension and other postretirement plan assets |  | 1,406 | 1,173 |
Assets measured at NAV | [1] | 1,004 | 832 |
Pension Benefits | Fixed-Income Securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Pension and other postretirement plan assets |  | 1,470 | 1,298 |
Assets measured at NAV | [1] | 618 | 463 |
Pension Benefits | Alternatives |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Pension and other postretirement plan assets |  | 637 | 586 |
Assets measured at NAV | [1] | 498 | 452 |
Pension Benefits | Other Investments |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Pension and other postretirement plan assets |  | 419 | 298 |
Assets measured at NAV | [1] | 91 | 96 |
Other Postretirement Benefits |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Pension and other postretirement plan assets |  | 54 | 55 | 56
Other Postretirement Benefits | Deposit Administration Fund |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Pension and other postretirement plan assets |  | 54 | 55 |
Level 1 | Pension Benefits |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Pension and other postretirement plan assets |  | 301 | 277 |
Level 1 | Pension Benefits | Equity Securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Pension and other postretirement plan assets |  | 269 | 230 |
Level 1 | Pension Benefits | Other Investments |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Pension and other postretirement plan assets |  | 32 | 47 |
Level 2 | Pension Benefits |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Pension and other postretirement plan assets |  | 1,091 | 1,003 |
Level 2 | Pension Benefits | Equity Securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Pension and other postretirement plan assets |  | 133 | 111 |
Level 2 | Pension Benefits | Fixed-Income Securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Pension and other postretirement plan assets |  | 834 | 824 |
Level 2 | Pension Benefits | Other Investments |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Pension and other postretirement plan assets |  | 124 | 68 |
Level 3 |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Pension and other postretirement plan assets |  | 383 | 287 | $ 208
Level 3 | Pension Benefits |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Pension and other postretirement plan assets |  | 329 | 232 |
Level 3 | Pension Benefits | Fixed-Income Securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Pension and other postretirement plan assets |  | 18 | 11 |
Level 3 | Pension Benefits | Alternatives |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Pension and other postretirement plan assets |  | 139 | 134 |
Level 3 | Pension Benefits | Other Investments |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Pension and other postretirement plan assets |  | 172 | 87 |
Level 3 | Other Postretirement Benefits | Deposit Administration Fund |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Pension and other postretirement plan assets |  | $ 54 | $ 55 |
[1] | In accordance with the relevant accounting standards, certain investments that are measured at fair value using the net asset value ("NAV") per share (or its equivalent) have not been classified in the fair value hierarchy. These investments are commingled funds that invest in fixed-income instruments including bonds, debt securities, and other similar instruments issued by various U.S. and non-U.S. public- or private-sector entities. Redemption periods for these investments range from daily to annually.

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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 + Details
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- Definition Net asset value of investment in certain entities that calculate net asset value per share. Includes, but is not limited to, by unit, membership interest, or other ownership interest. Investment includes, but is not limited to, investment in certain hedge funds, venture capital funds, private equity funds, real estate partnerships or funds. Excludes fair value disclosure. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 6A -URI http://asc.fasb.org/extlink&oid=66048111&loc=SL6742756-110258 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 35 -Paragraph 59 -URI http://asc.fasb.org/extlink&oid=65894324&loc=SL6740821-110257 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 35 -Paragraph 54B -URI http://asc.fasb.org/extlink&oid=65894324&loc=SL7495116-110257 + Details
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