v3.8.0.1
Employee Benefit Plans (Effects of a One-Percentage Point Increase and Decrease in Assumed Health Care Cost Trend Rates) (Details) - Other Postretirement Benefits
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Defined Benefit Plan Disclosure [Line Items]  
Increase in total service and interest costs $ 7
Increase in postretirement benefit obligation 130
Decrease in total service and interest costs 5
Decrease in postretirement benefit obligation $ 98