Benefit Plans - Components of net periodic cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension benefits
Components of net periodic cost
Service cost	$ 113	$ 114	$ 113
Interest cost	242	264	286
Expected return on plan assets	(421)	(409)	(398)
Amortization of:
Prior service credit	(56)	(56)	(56)
Net actuarial loss (gain)	176	189	174
Settlement loss	203	153	27
Net periodic cost (credit)	257	255	146
Postretirement benefits
Components of net periodic cost
Service cost	7	8	9
Interest cost	13	15	17
Expected return on plan assets	0	0	0
Amortization of:
Prior service credit	(21)	(25)	(21)
Net actuarial loss (gain)	(22)	(24)	(24)
Settlement loss	0	0	0
Net periodic cost (credit)	$ (23)	$ (26)	$ (19)