Pension and Other Postretirement Benefits - Assumptions Used and Trend Rates (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assumed health care cost trend rates of postretirement benefit obligation
Health care cost trend rate assumed for the next year	6.25%	6.00%	6.00%
Ultimate trend rate	4.50%	4.50%	4.50%
Year the rate reaches the ultimate trend rate	2025	2023	2021
One-percentage-point change in assumed health care cost trend rates
1-Percentage-Point Increase - Change in service cost and interest cost	$ 0
1-Percentage-Point Decrease - Change in service cost and interest cost	(1)
1-Percentage-Point Increase - Change in postretirement benefit obligation	6
1-Percentage-Point Decrease - Change in postretirement benefit obligation	$ (11)
Pension
Assumptions used to determine benefit obligations at December 31:
Discount rate	3.12%	3.41%	3.95%
Rate of compensation increases	3.54%	3.77%	3.72%
Assumptions used to determine net periodic benefit cost for years ended December 31:
Discount rate	3.41%	3.95%	3.70%
Expected return on plan assets	5.53%	6.22%	6.54%
Rate of compensation increases	3.77%	3.72%	3.72%
Other Postretirement Benefits
Assumptions used to determine benefit obligations at December 31:
Discount rate	3.72%	4.30%	4.55%
Rate of compensation increases
Assumptions used to determine net periodic benefit cost for years ended December 31:
Discount rate	4.30%	4.55%	4.15%
Expected return on plan assets	6.80%	7.00%	7.00%
Rate of compensation increases