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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Components of net periodic cost
Costs and expenses	$ (40)	$ (23)	$ (95)	$ (53)
Remeasurement (gains) losses	(71)	225	320	365
Total net (benefit) cost	(111)	202	225	312
Pension benefits
Components of net periodic cost
Service cost	25	30	78	86
Interest cost	49	58	161	185
Expected return on plan assets	(104)	(103)	(303)	(297)
Amortization of prior service credit	(14)	(14)	(42)	(42)
Curtailment gain (loss)	10	0	10	0
Costs and expenses	(34)	(29)	(96)	(68)
Remeasurement of projected benefit obligation	130	323	686	1,054
Remeasurement of plan assets	(202)	(136)	(391)	(752)
Remeasurement (gains) losses	(72)	187	295	302
Total net (benefit) cost	(106)	158	199	234
Postretirement benefits
Components of net periodic cost
Service cost	1	2	4	6
Interest cost	2	4	8	11
Amortization of prior service credit	(1)	0	(3)	(2)
Curtailment gain (loss)	(8)	0	(8)	0
Costs and expenses	(6)	6	1	15
Remeasurement of projected benefit obligation	1	38	25	63
Remeasurement of plan assets	0	0	0	0
Remeasurement (gains) losses	1	38	25	63
Total net (benefit) cost	$ (5)	$ 44	$ 26	$ 78