Employee Benefits - Benefit Obligations Actuarial Assumptions (Details)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Ultimate health care cost trend rate	4.50%
Minimum
Defined Benefit Plan Disclosure [Line Items]
Initial health care cost trend rate	5.00%
Maximum
Defined Benefit Plan Disclosure [Line Items]
Initial health care cost trend rate	7.00%
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.29%	3.57%
Rate of compensation increase	3.25%	3.25%
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.26%	3.53%
Ultimate health care cost trend rate	4.50%	4.50%	4.50%
Other Postretirement Benefits Plan | Minimum
Defined Benefit Plan Disclosure [Line Items]
Initial health care cost trend rate	5.00%	5.00%
Other Postretirement Benefits Plan | Maximum
Defined Benefit Plan Disclosure [Line Items]
Initial health care cost trend rate	7.00%	7.00%