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Pension and Other Postemployment Benefits (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Amounts Recognized on the Balance Sheet: |  |  |
Noncurrent liabilities | $ (3,975) | $ (3,719) |
Pension |  |  |
Change in benefit obligation: |  |  |
Benefit obligation as of January 1 | 9,426 | 9,079 |
Service cost | 208 | 191 | $ 193
Interest cost | 390 | 398 | 383
Plan participants’ contributions | 0 | 0 |
Plan amendments | 11 | 0 |
Medicare retiree drug subsidy | 0 | 0 |
Actuarial loss | 610 | 224 |
Benefits paid | (478) | (466) |
Benefit obligation as of December 31 | 10,167 | 9,426 | 9,079
Change in fair value of plan assets: |  |  |
Fair value of plan assets as of January 1 | 6,213 | 5,338 |
Actual return on plan assets | 950 | 442 |
Company contributions | 18 | 899 |
Plan participants’ contributions | 0 | 0 |
Benefits paid | (477) | (466) |
Fair value of plan assets as of December 31 | 6,704 | 6,213 | 5,338
Funded Status: |  |  |
Funded Status | (3,463) | (3,213) |
Accumulated benefit obligation | 9,583 | 8,913 |
Amounts Recognized on the Balance Sheet: |  |  |
Noncurrent assets | 0 | 9 |
Current liabilities | (19) | (19) |
Noncurrent liabilities | (3,444) | (3,203) |
Net liability as of December 31 | (3,463) | (3,213) |
Amounts Recognized in AOCI: |  |  |
Prior service cost (credit) | $ 32 | $ 28 |
Assumptions Used to Determine Benefit Obligations |  |  |
Discount rate | 3.75% | 4.25% |
Rate of compensation increase | 4.20% | 4.20% |
Allocation of Plan Assets |  |  |
Asset Allocation | 100.00% | 100.00% |
Pension | Equity securities |  |  |
Allocation of Plan Assets |  |  |
Asset Allocation | 42.00% | 44.00% |
Pension | Bonds |  |  |
Allocation of Plan Assets |  |  |
Asset Allocation | 32.00% | 30.00% |
Pension | Absolute return strategies |  |  |
Allocation of Plan Assets |  |  |
Asset Allocation | 10.00% | 8.00% |
Pension | Real estate |  |  |
Allocation of Plan Assets |  |  |
Asset Allocation | 9.00% | 10.00% |
Pension | Private equity funds |  |  |
Allocation of Plan Assets |  |  |
Asset Allocation | 1.00% | 0.00% |
Pension | Cash and short-term securities |  |  |
Allocation of Plan Assets |  |  |
Asset Allocation | 6.00% | 8.00% |
Pension | Qualified plan |  |  |
Funded Status: |  |  |
Funded Status | $ (3,043) | $ (2,821) |
Pension | Non-qualified plans |  |  |
Funded Status: |  |  |
Funded Status | (420) | (392) |
OPEB |  |  |
Change in benefit obligation: |  |  |
Benefit obligation as of January 1 | 711 | 724 |
Service cost | 5 | 5 | 5
Interest cost | 27 | 30 | 29
Plan participants’ contributions | 4 | 5 |
Plan amendments | 0 | (13) |
Medicare retiree drug subsidy | 1 | 1 |
Actuarial loss | 32 | 14 |
Benefits paid | (49) | (55) |
Benefit obligation as of December 31 | 731 | 711 | 724
Change in fair value of plan assets: |  |  |
Fair value of plan assets as of January 1 | 420 | 431 |
Actual return on plan assets | 49 | 30 |
Company contributions | 16 | 9 |
Plan participants’ contributions | 4 | 5 |
Benefits paid | (50) | (55) |
Fair value of plan assets as of December 31 | 439 | 420 | $ 431
Funded Status: |  |  |
Funded Status | (292) | (291) |
Accumulated benefit obligation | 0 | 0 |
Amounts Recognized on the Balance Sheet: |  |  |
Noncurrent assets | 0 | 0 |
Current liabilities | 0 | 0 |
Noncurrent liabilities | (292) | (291) |
Net liability as of December 31 | (292) | (291) |
Amounts Recognized in AOCI: |  |  |
Prior service cost (credit) | $ (206) | $ (288) |
Assumptions Used to Determine Benefit Obligations |  |  |
Discount rate | 3.50% | 4.00% |
Rate to which the cost trend rate is assumed to decline (the ultimate trend rate) | 4.50% | 4.50% |
Allocation of Plan Assets |  |  |
Asset Allocation | 100.00% | 100.00% |
OPEB | Equity securities |  |  |
Allocation of Plan Assets |  |  |
Asset Allocation | 50.00% | 53.00% |
OPEB | Bonds |  |  |
Allocation of Plan Assets |  |  |
Asset Allocation | 33.00% | 41.00% |
OPEB | Absolute return strategies |  |  |
Allocation of Plan Assets |  |  |
Asset Allocation | 0.00% | 0.00% |
OPEB | Real estate |  |  |
Allocation of Plan Assets |  |  |
Asset Allocation | 0.00% | 0.00% |
OPEB | Private equity funds |  |  |
Allocation of Plan Assets |  |  |
Asset Allocation | 0.00% | 0.00% |
OPEB | Cash and short-term securities |  |  |
Allocation of Plan Assets |  |  |
Asset Allocation | 17.00% | 6.00% |
OPEB | Pre Medicare |  |  |
Assumptions Used to Determine Benefit Obligations |  |  |
Health care cost trend rate assumed (pre/post-Medicare) | 6.00% | 6.00% |
OPEB | Post Medicare |  |  |
Assumptions Used to Determine Benefit Obligations |  |  |
Health care cost trend rate assumed (pre/post-Medicare) | 5.50% | 5.50% |

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- Definition Amount of actuarial present value of benefits attributed to service rendered by employee for defined benefit plan. + 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 (a) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39716-114964 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(6) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount, before benefit payment, of receipt from prescription drug subsidy which increases benefit obligation of defined benefit postretirement plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39716-114964 + Details
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- Definition Amount of contribution received by defined benefit plan from employer which increases plan assets. + 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)(ii) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2709-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 4: 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)(01) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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