Pension and Other Postretirement Benefits - Assumptions Used and Trend Rates (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assumed health care cost trend rates of postretirement benefit obligation
Health care cost trend rate assumed for the next year	6.70%	7.00%	6.25%
Ultimate trend rate	4.50%	4.50%	4.50%
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	2
1-Percentage-Point Decrease - Change in postretirement benefit obligation	$ (3)
Pension
Assumptions used to determine benefit obligations as of December 31:
Discount rate	2.61%	3.66%	3.12%
Rate of compensation increases	3.44%	3.52%	3.54%
Assumptions used to determine net periodic benefit cost for the twelve months ended December 31:
Discount rate	3.66%	3.12%	3.41%
Expected return on plan assets	4.71%	4.77%	5.53%
Rate of compensation increases	3.52%	3.54%	3.77%
Other Postretirement Benefits
Assumptions used to determine benefit obligations as of December 31:
Discount rate	3.29%	4.40%	3.72%
Assumptions used to determine net periodic benefit cost for the twelve months ended December 31:
Discount rate	4.40%	3.72%	4.30%
Expected return on plan assets	6.70%	6.80%	6.80%