Benefit Plans - Components of net periodic cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Components of net periodic cost
Service cost	$ 108	$ 125	$ 117
Interest cost	221	254	270
Expected return on plan assets	(414)	(403)	(427)
Amortization of prior service credit	(64)	(59)	(77)
Curtailment losses (gains)	2	0	0
Costs and expenses	(147)	(83)	(117)
Remeasurement of projected benefit obligation	835	946	(259)
Remeasurement of plan assets	(886)	(832)	727
Remeasurement (gains) losses	(51)	114	468
Total net (benefit) cost	(198)	31	351
Pension benefits
Components of net periodic cost
Service cost	104	117	110
Interest cost	210	240	255
Expected return on plan assets	(414)	(403)	(427)
Amortization of prior service credit	(54)	(56)	(56)
Curtailment losses (gains)	10	0	0
Costs and expenses	(144)	(102)	(118)
Remeasurement of projected benefit obligation	813	927	(255)
Remeasurement of plan assets	(886)	(832)	727
Remeasurement (gains) losses	(73)	95	472
Total net (benefit) cost	(217)	(7)	354
Postretirement benefits
Components of net periodic cost
Service cost	4	8	7
Interest cost	11	14	15
Expected return on plan assets	0	0	0
Amortization of prior service credit	(10)	(3)	(21)
Curtailment losses (gains)	(8)	0	0
Costs and expenses	(3)	19	1
Remeasurement of projected benefit obligation	22	19	(4)
Remeasurement of plan assets	0	0	0
Remeasurement (gains) losses	22	19	(4)
Total net (benefit) cost	$ 19	$ 38	$ (3)