Pension and Other Postretirement Employee Benefit Plans - Defined Benefit Pension Plans (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in projected and accumulated benefit obligations
Benefit obligation, beginning of year	$ (17,705,000,000)
Benefit obligation, end of year	(19,137,000,000)	$ (17,705,000,000)
Change in plan assets
Fair value of plan assets, beginning of year	23,366,000,000
Fair value of plan assets, end of year	25,417,000,000	23,366,000,000
Net funded status	6,280,000,000	5,661,000,000
Amounts recorded in accumulated other comprehensive income/(loss)
Accumulated other comprehensive income/(loss)	(1,586,000,000)	(1,816,000,000)
Defined benefit pension plans | U.S.
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plan, estimated future employer contributions in next fiscal year	0
Change in projected and accumulated benefit obligations
Benefit obligation, beginning of year	(13,277,000,000)	(12,173,000,000)
Benefits earned during the year	(2,000,000)	(327,000,000)	$ (323,000,000)
Interest cost on benefit obligations	(422,000,000)	(518,000,000)	(478,000,000)
Plan amendments	0	(5,000,000)
Net gain/(loss)	(1,086,000,000)	(944,000,000)
Benefits paid	640,000,000	690,000,000
Benefit obligation, end of year	(14,147,000,000)	(13,277,000,000)	(12,173,000,000)
Change in plan assets
Fair value of plan assets, beginning of year	16,329,000,000	14,521,000,000
Actual return on plan assets	1,901,000,000	2,465,000,000
Firm contributions	29,000,000	33,000,000
Benefits paid	(640,000,000)	(690,000,000)
Fair value of plan assets, end of year	17,619,000,000	16,329,000,000	$ 14,521,000,000
Net funded status	3,472,000,000	3,052,000,000
Amounts recorded in accumulated other comprehensive income/(loss)
Net gain/(loss)	(1,558,000,000)	(1,745,000,000)
Prior service credit/(cost)	(4,000,000)	(5,000,000)
Accumulated other comprehensive income/(loss)	$ (1,562,000,000)	$ (1,750,000,000)
Weighted-average actuarial assumptions used to value benefit obligations
Discount rate	2.50%	3.30%
Interest crediting rate	4.65%	4.65%
Defined benefit pension plans | Non-U.S.
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plan, estimated future employer contributions in next fiscal year	$ 50,000,000
Defined benefit plan, contractually required future employer contributions in next fiscal year	35,000,000
OPEB plans
Change in projected and accumulated benefit obligations
Benefit obligation, beginning of year	(4,428,000,000)
Benefit obligation, end of year	(4,990,000,000)	$ (4,428,000,000)
Change in plan assets
Fair value of plan assets, beginning of year	7,037,000,000
Fair value of plan assets, end of year	7,798,000,000	7,037,000,000
Net funded status	2,808,000,000	2,609,000,000
Amounts recorded in accumulated other comprehensive income/(loss)
Accumulated other comprehensive income/(loss)	$ (24,000,000)	$ (66,000,000)