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%

Defined Benefit Plan, Assets, Actual Allocations [Abstract]

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Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level.

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Amount, before tax, of accumulated other comprehensive (income) loss for cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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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.

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Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan.

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Amount of asset, recognized in statement of financial position, for overfunded defined benefit pension and other postretirement plans.

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Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan.

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Amount, before benefit payment, of receipt from prescription drug subsidy which increases benefit obligation of defined benefit postretirement plan.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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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.

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Amount of funded (unfunded) status of defined benefit plan, measured as difference between fair value of plan assets and benefit obligation. Includes, but is not limited to, overfunded (underfunded) status.

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Assumed rate, for next fiscal year, based on annual change in cost of health care cost benefits used to measure expected cost of benefits covered by defined benefit postretirement plan. Factors include, but are not limited to, estimate of health care inflation, change in health care utilization or delivery pattern, technological advances, and change in health status of participant. Excludes factors for change in composition of plan population by age and dependency status.

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Ultimate trend rate for health care cost for defined benefit postretirement plan.

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Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held.

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Amount of liability, recognized in statement of financial position, for defined benefit pension and other postretirement plans, classified as current.

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- Details

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