Benefit Plans - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Components of net periodic cost
Remeasurement gains and (losses)	$ (225)	$ 39	$ (365)	$ 32
Remeasurement due to change in weighted average discount rate	324		1,040
(Increase) decrease for remeasurement due to settlement	37	$ 3	81	$ 72
Increase (decrease) for actual return (loss)	$ 136		$ 752
Pension benefits
Components of net periodic cost
Discount rate	3.17%		3.17%		3.53%	3.87%	4.30%