Pension and Postretirement Benefit Plans and Defined Contribution Plans - One-Percentage-Point Increase or Decrease in the Healthcare Cost Trend Rate (Details) - Postretirement Benefits Plan [Member] $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Effect on total service and interest cost components, increase	$ 3
Effect on total service and interest cost components, decrease	(4)
Effect on postretirement benefit obligation, increase	47
Effect on postretirement benefit obligation, decrease	$ (26)