Benefit Plans - Components of net cost (benefit) for pension and other postretirement plans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Components of net periodic cost
Costs and expenses	$ (62)	$ (31)
Remeasurement (gains) losses	(310)	318
Pension net (benefit) cost	(372)	287
Pension benefits
Components of net periodic cost
Service cost	26	28
Interest cost	46	56
Expected return on plan assets	(117)	(104)
Amortization of prior service credit	(13)	(14)
Costs and expenses	(58)	(34)
Remeasurement of projected benefit obligation	(512)	(116)
Remeasurement of plan assets	221	434
Remeasurement (gains) losses	(291)	318
Pension net (benefit) cost	(349)	284
Postretirement benefits
Components of net periodic cost
Service cost	0	1
Interest cost	2	3
Amortization of prior service credit	(6)	(1)
Costs and expenses	(4)	3
Remeasurement of projected benefit obligation	(19)	0
Remeasurement of plan assets	0	0
Remeasurement (gains) losses	(19)	0
Pension net (benefit) cost	$ (23)	$ 3