Benefit Plans - Components of net periodic cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension benefits
Components of net periodic cost
Service cost	$ 114	$ 113	$ 114
Interest cost	264	286	258
Expected return on plan assets	(409)	(398)	(424)
Amortization of:
Prior service credit	(56)	(56)	(56)
Net actuarial loss (gain)	189	174	190
Settlement loss	153	27	31
Net periodic cost (credit)	255	146	113
Postretirement benefits
Components of net periodic cost
Service cost	8	9	12
Interest cost	15	17	23
Expected return on plan assets	0	0	0
Amortization of:
Prior service credit	(25)	(21)	(22)
Net actuarial loss (gain)	(24)	(24)	(9)
Settlement loss	0	0	0
Net periodic cost (credit)	$ (26)	$ (19)	$ 4