Benefit Plans - Changes not yet recognized (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 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 cost – December 31, 2018	$ (198)
Prior service credit amortized to net cost	56
Items not yet recognized as a component of net cost – December 31, 2019	(142)
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 cost – December 31, 2018	(16)
Prior service credit amortized to net cost	3
Items not yet recognized as a component of net cost – December 31, 2019	$ (13)