Employee Benefit Plans - Summary of Change in Benefit Obligation and Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
U.S.
Change in Plan Assets
Fair value of plan assets at January 1	$ 9,948
Employer contributions	803
Fair value of plan assets at December 31	8,532	$ 9,948
Int’l.
Change in Plan Assets
Fair value of plan assets at January 1	4,766
Employer contributions	232
Fair value of plan assets at December 31	4,142	4,766
Pension Benefits \| U.S.
Change in Benefit Obligation
Benefit obligation at January 1	13,580	13,271
Service cost	480	489	$ 494
Interest cost	370	366	377
Plan participants' contributions	0	0
Plan amendments	0	0
Actuarial (gain) loss	(1,051)	1,168
Foreign currency exchange rate changes	0	0
Benefits paid	(1,653)	(1,714)
Divestitures	0	0
Benefit obligation at December 31	11,726	13,580	13,271
Change in Plan Assets
Fair value of plan assets at January 1	9,948	9,550
Actual return on plan assets	(566)	1,384
Foreign currency exchange rate changes	0	0
Employer contributions	803	728
Plan participants' contributions	0	0
Benefits paid	(1,653)	(1,714)
Divestitures	0	0
Fair value of plan assets at December 31	8,532	9,948	9,550
Funded status at December 31	(3,194)	(3,632)
Pension Benefits \| Int’l.
Change in Benefit Obligation
Benefit obligation at January 1	5,540	5,169
Service cost	141	151	159
Interest cost	206	219	261
Plan participants' contributions	4	4
Plan amendments	23	1
Actuarial (gain) loss	(239)	(37)
Foreign currency exchange rate changes	(227)	374
Benefits paid	(432)	(310)
Divestitures	(196)	(31)
Benefit obligation at December 31	4,820	5,540	5,169
Change in Plan Assets
Fair value of plan assets at January 1	4,766	4,174
Actual return on plan assets	(9)	319
Foreign currency exchange rate changes	(221)	358
Employer contributions	232	252
Plan participants' contributions	4	4
Benefits paid	(432)	(310)
Divestitures	(198)	(31)
Fair value of plan assets at December 31	4,142	4,766	4,174
Funded status at December 31	(678)	(774)
Other Benefits
Change in Benefit Obligation
Benefit obligation at January 1	2,788	2,549
Service cost	42	32	60
Interest cost	94	95	128
Plan participants' contributions	71	78
Plan amendments	2	0
Actuarial (gain) loss	(272)	266
Foreign currency exchange rate changes	(9)	10
Benefits paid	(237)	(229)
Divestitures	(49)	(13)
Benefit obligation at December 31	2,430	2,788	2,549
Change in Plan Assets
Fair value of plan assets at January 1	0	0
Actual return on plan assets	0	0
Foreign currency exchange rate changes	0	0
Employer contributions	166	151
Plan participants' contributions	71	78
Benefits paid	(237)	(229)
Divestitures	0	0
Fair value of plan assets at December 31	0	0	$ 0
Funded status at December 31	$ (2,430)	$ (2,788)

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 actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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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.

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Amount of contributions received by defined benefit plan from participant which increase benefit obligation.

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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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Amount of contribution received by defined benefit plan from employer which increases plan assets.

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Amount of decrease in benefit obligation of defined benefit plan from disposal or classification as held-for-sale.

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Amount of decrease in plan assets of defined benefit plan from disposal or classification as held-for-sale.

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Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee.

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Amount of foreign currency translation gain (loss) which (increases) decreases benefit obligation of defined benefit plan.

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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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Amount of cost recognized for passage of time related to defined benefit plan.

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Amount of increase (decrease) in benefit obligation of defined benefit plan from change in terms of existing plan or initiation of new plan.

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Amount of payment to participant under defined benefit plan which decreases plan assets. 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.

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Amount of foreign currency translation gain (loss) which increases (decreases) plan assets of defined benefit plan.

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Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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