Benefit Plans - Components of net cost (benefit) for pension and other postretirement plans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Components of net periodic cost
Costs and expenses	$ (48)	$ (24)	$ (110)	$ (55)
Remeasurement (gains) losses	(134)	73	(444)	391
Pension net (benefit) cost	(182)	49	(554)	336
Pension benefits
Components of net periodic cost
Service cost	25	25	51	53
Interest cost	52	56	98	112
Expected return on plan assets	(109)	(95)	(226)	(199)
Amortization of prior service credit	(12)	(14)	(25)	(28)
Costs and expenses	(44)	(28)	(102)	(62)
Remeasurement of projected benefit obligation	245	672	(267)	556
Remeasurement of plan assets	(388)	(623)	(167)	(189)
Remeasurement (gains) losses	(143)	49	(434)	367
Pension net (benefit) cost	(187)	21	(536)	305
Postretirement benefits
Components of net periodic cost
Service cost	1	2	1	3
Interest cost	2	3	4	6
Amortization of prior service credit	(7)	(1)	(13)	(2)
Costs and expenses	(4)	4	(8)	7
Remeasurement of projected benefit obligation	9	24	(10)	24
Remeasurement of plan assets	0	0	0	0
Remeasurement (gains) losses	9	24	(10)	24
Pension net (benefit) cost	$ 5	$ 28	$ (18)	$ 31