Retirement Benefits (Details 5) $ in Millions	Dec. 31, 2017 USD ($)
Amounts expected to be recognized in 2018 net periodic benefit cost
Net actuarial loss	$ 535
Prior service credit	(79)
Pension Benefits
Amounts expected to be recognized in 2018 net periodic benefit cost
Net actuarial loss	535
Prior service credit	(58)
Medical and Life Benefits
Amounts expected to be recognized in 2018 net periodic benefit cost
Net actuarial loss	0
Prior service credit	$ (21)