Benefit Plans - Funded status (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Items not yet recognized as a component of net periodic cost:
Deferred income tax	$ (425)	$ (782)
Unrecognized pension and other postretirement benefit cost	1,491	1,347
Pension benefits
Components of net periodic cost
Fair value of plan assets	5,299	6,284	$ 5,650
Less: Benefit obligation	6,224	6,815	6,591
Funded status	(925)	(531)
Items not yet recognized as a component of net periodic cost:
Net actuarial loss (gain)	2,313	2,224
Prior service credit	(198)	(254)
Unrecognized pension and other postretirement benefit cost, pre-tax	2,115	1,970
Deferred income tax	(449)	(419)
Unrecognized pension and other postretirement benefit cost	1,666	1,551
Postretirement benefits
Components of net periodic cost
Fair value of plan assets	0	0
Less: Benefit obligation	375	386	$ 373
Funded status	(375)	(386)
Items not yet recognized as a component of net periodic cost:
Net actuarial loss (gain)	(200)	(218)
Prior service credit	(16)	(37)
Unrecognized pension and other postretirement benefit cost, pre-tax	(216)	(255)
Deferred income tax	41	51
Unrecognized pension and other postretirement benefit cost	$ (175)	$ (204)