Benefit Plans - Future amortization amounts (Details) $ in Millions	Dec. 31, 2017 USD ($)
Pension benefits
Estimates of the net actuarial loss (gain) and prior service credit expected to be recognized as a component of net periodic benefit cost during 2015
Net actuarial loss (gain)	$ 177
Prior service credit	(56)
Postretirement benefits
Estimates of the net actuarial loss (gain) and prior service credit expected to be recognized as a component of net periodic benefit cost during 2015
Net actuarial loss (gain)	(22)
Prior service credit	$ (22)