Employee Benefit Plans - Summary of Change in Benefit Obligation and Plan Assets (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
U.S.
Change in Plan Assets
Fair value of plan assets at January 1		$ 8,532
Employer contributions		1,096
Fair value of plan assets at December 31		10,177	$ 8,532
Int’l.
Change in Plan Assets
Fair value of plan assets at January 1		4,142
Employer contributions		266
Fair value of plan assets at December 31		4,791	4,142
Pension Benefits \| U.S.
Change in Benefit Obligation
Benefit obligation at January 1		11,726	13,580
Service cost		406	480	$ 489
Interest cost		397	370	366
Plan participants’ contributions		0	0
Plan amendments		0	0
Actuarial (gain) loss		2,922	(1,051)
Foreign currency exchange rate changes		0	0
Benefits paid		(1,035)	(1,653)
Curtailment	$ 0	0
Acquisitions		49
Divestitures/Acquisitions			0
Benefit obligation at December 31		14,465	11,726	13,580
Change in Plan Assets
Fair value of plan assets at January 1		8,532	9,948
Actual return on plan assets		1,548	(566)
Foreign currency exchange rate changes		0	0
Employer contributions		1,096	803
Plan participants’ contributions		0	0
Benefits paid		(1,035)	(1,653)
Acquisitions		36
Divestitures/Acquisitions			0
Fair value of plan assets at December 31		10,177	8,532	9,948
Funded status at December 31		(4,288)	(3,194)
Pension Benefits \| Int’l.
Change in Benefit Obligation
Benefit obligation at January 1		4,820	5,540
Service cost		139	141	151
Interest cost		199	206	219
Plan participants’ contributions		4	4
Plan amendments		29	23
Actuarial (gain) loss		673	(239)
Foreign currency exchange rate changes		121	(227)
Benefits paid		(302)	(432)
Curtailment	0	(3)
Acquisitions		0
Divestitures/Acquisitions			(196)
Benefit obligation at December 31		5,680	4,820	5,540
Change in Plan Assets
Fair value of plan assets at January 1		4,142	4,766
Actual return on plan assets		566	(9)
Foreign currency exchange rate changes		115	(221)
Employer contributions		266	232
Plan participants’ contributions		4	4
Benefits paid		(302)	(432)
Acquisitions		0
Divestitures/Acquisitions			(198)
Fair value of plan assets at December 31		4,791	4,142	4,766
Funded status at December 31		(889)	(678)
Other Benefits
Change in Benefit Obligation
Benefit obligation at January 1		2,430	2,788
Service cost		36	42	32
Interest cost		96	94	95
Plan participants’ contributions		72	71
Plan amendments		0	2
Actuarial (gain) loss		125	(272)
Foreign currency exchange rate changes		2	(9)
Benefits paid		(240)	(237)
Curtailment	$ 0	0
Divestitures/Acquisitions		(1)	(49)
Benefit obligation at December 31		2,520	2,430	2,788
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		168	166
Plan participants’ contributions		72	71
Benefits paid		(240)	(237)
Divestitures/Acquisitions		0	0
Fair value of plan assets at December 31		0	0	$ 0
Funded status at December 31		$ (2,520)	$ (2,430)

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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Amount of increase in benefit obligation of defined benefit plan from business combination.

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Amount of increase in plan assets of defined benefit plan from business combination.

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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 (increase) decrease in benefit obligation of defined benefit plan from event reducing expected years of future service of present employees or eliminating accrual of benefits for some or all future services of present employees.

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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 contributions received by defined benefit plan from participant which increases plan assets.

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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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