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Reconciliation of Change in Projected Benefit Obligation and Plan Assets (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Pension Benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Accumulated benefit obligation: | $ 4,739 | $ 4,158 |
Benefit obligation at beginning of year | 5,253 | 4,473 |
Service cost | 195 | 112 | $ 124
Interest cost | 220 | 200 | 200
Actuarial (gain) loss | 525 | 738 |
Gross benefits paid and settlements | (366) | (243) |
Other | 25 | (27) |
Benefit obligation at end of year | 5,852 | 5,253 | 4,473
Fair value of plan assets at beginning of year | 3,355 | 2,975 |
Actual return on plan assets | 510 | 230 |
Employer contributions | 419 | 421 |
Gross benefits paid and settlements | (366) | (243) |
Other | 14 | (28) |
Fair value of plan assets at end of year | 3,932 | 3,355 | 2,975
Funded status-Net amount recognized | (1,920) | (1,898) |
Other Postretirement Benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Benefit obligation at beginning of year | 1,687 | 2,002 |
Service cost | 13 | 19 | 21
Interest cost | 66 | 86 | 82
Plan participants' contributions | 68 | 69 |
Benefits paid | (178) | (191) |
Actuarial (gain) loss | 40 | (165) |
Plan amendments |  | (138) |
Other | 14 | 5 |
Benefit obligation at end of year | 1,710 | 1,687 | 2,002
Fair value of plan assets at beginning of year | 55 | 56 |
Actual return on plan assets | 1 | 2 |
Employer contributions | 108 | 119 |
Plan participants' contributions | 68 | 69 |
Benefits paid | (178) | (191) |
Fair value of plan assets at end of year | 54 | 55 | $ 56
Funded status-Net amount recognized | $ (1,656) | $ (1,632) |

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- Definition Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level. + 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 (e) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses. + 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)(1) -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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