Benefit Plans - Components of net periodic cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Components of net periodic cost
Service cost	$ 125	$ 117	$ 119
Interest cost	254	270	269
Expected return on plan assets	(403)	(427)	(419)
Amortization of prior service credit	(59)	(77)	(81)
Costs and expenses	(83)	(117)	(112)
Remeasurement of projected benefit obligation	946	(259)	414
Remeasurement of plan assets	(832)	727	(631)
Remeasurement gains and losses	114	468	(217)
Total net (benefit) cost	31	351	(329)
Pension benefits
Components of net periodic cost
Service cost	117	110	111
Interest cost	240	255	254
Expected return on plan assets	(403)	(427)	(419)
Amortization of prior service credit	(56)	(56)	(56)
Costs and expenses	(102)	(118)	(110)
Remeasurement of projected benefit obligation	927	(255)	406
Remeasurement of plan assets	(832)	727	(631)
Remeasurement gains and losses	95	472	(225)
Total net (benefit) cost	(7)	354	(335)
Postretirement benefits
Components of net periodic cost
Service cost	8	7	8
Interest cost	14	15	15
Expected return on plan assets	0	0	0
Amortization of prior service credit	(3)	(21)	(25)
Costs and expenses	19	1	(2)
Remeasurement of projected benefit obligation	19	(4)	8
Remeasurement of plan assets	0	0	0
Remeasurement gains and losses	19	(4)	8
Total net (benefit) cost	$ 38	$ (3)	$ 6