Benefit Plans - Components of net cost (benefit) for pension and other postretirement plans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Components of net periodic cost
Curtailment gain (loss)	$ 0	$ (8)	$ 0	$ (8)
Costs and expenses	(54)	(40)	(164)	(95)
Remeasurement (gains) losses	40	(71)	(404)	320
Pension net (benefit) cost	(14)	(111)	(568)	225
Pension benefits
Components of net periodic cost
Service cost	27	25	78	78
Interest cost	47	49	145	161
Expected return on plan assets	(112)	(104)	(338)	(303)
Amortization of prior service credit	(13)	(14)	(38)	(42)
Curtailment gain (loss)	0	10	0	10
Costs and expenses	(51)	(34)	(153)	(96)
Remeasurement of projected benefit obligation	(25)	130	(292)	686
Remeasurement of plan assets	68	(202)	(99)	(391)
Remeasurement (gains) losses	43	(72)	(391)	295
Pension net (benefit) cost	(8)	(106)	(544)	199
Postretirement benefits
Components of net periodic cost
Service cost	0	1	1	4
Interest cost	2	2	6	8
Amortization of prior service credit	(5)	(1)	(18)	(3)
Curtailment gain (loss)	0	(8)	0	(8)
Costs and expenses	(3)	(6)	(11)	1
Remeasurement of projected benefit obligation	(3)	1	(13)	25
Remeasurement of plan assets	0	0	0	0
Remeasurement (gains) losses	(3)	1	(13)	25
Pension net (benefit) cost	$ (6)	$ (5)	$ (24)	$ 26