Benefit Plans - Changes not yet recognized (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Pension benefits
Change in items not yet recognized as a component of defined benefit plans, net periodic cost
Items not yet recognized as a component of net periodic cost – December 31, 2016	$ 2,497
Net actuarial (gain) loss arising during the period	(247)
Net actuarial (loss) gain amortized to net periodic benefit cost	(342)
Prior service credit amortized to net periodic benefit cost	56
Translation adjustment and other	6
Items not yet recognized as a component of net periodic cost – December 31, 2017	1,970
Postretirement benefits
Change in items not yet recognized as a component of defined benefit plans, net periodic cost
Items not yet recognized as a component of net periodic cost – December 31, 2016	(313)
Net actuarial (gain) loss arising during the period	8
Net actuarial (loss) gain amortized to net periodic benefit cost	24
Prior service credit amortized to net periodic benefit cost	25
Translation adjustment and other	1
Items not yet recognized as a component of net periodic cost – December 31, 2017	$ (255)