Benefit Plans - Changes in benefit obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in benefit obligation
Service cost	$ 108	$ 125	$ 117
Interest cost	221	254	270
Pension benefits
Change in benefit obligation
Benefit obligation, beginning of year	7,139	6,224
Service cost	104	117	110
Interest cost	210	240	255
Participant contributions	0	0
Actuarial losses (gains)	813	927
Benefits paid	(522)	(356)
Plan amendments	0	0
Translation adjustment and other	(1)	(13)
Curtailment losses (gains)	20	0
Benefit obligation, end of year	7,763	7,139	6,224
Change in plan assets
Fair value of plan assets, beginning of year	6,192	5,299
Actual return on plan assets	1,300	1,235
Employer contribution	18	27
Benefits paid	(522)	(356)
Translation adjustment and other	(1)	(13)
Fair value of plan assets, end of year	6,987	6,192	5,299
Funded status	(776)	(947)
Prior service credit	(78)
Unamortized pension and other postretirement prior service credit	(78)	(142)
Postretirement benefits
Change in benefit obligation
Benefit obligation, beginning of year	397	375
Service cost	4	8	7
Interest cost	11	14	15
Participant contributions	14	15
Actuarial losses (gains)	22	19
Benefits paid	(37)	(39)
Plan amendments	(102)	0
Translation adjustment and other	(1)	5
Curtailment losses (gains)	10	0
Benefit obligation, end of year	318	397	$ 375
Change in plan assets
Fair value of plan assets, beginning of year
Employer contribution	23	24
Fair value of plan assets, end of year
Funded status	(318)	(397)
Prior service credit	(89)
Unamortized pension and other postretirement prior service credit	$ (89)	$ (13)