Employee Benefit Plans - Postretirement Plan Assumptions (Details) - Postretirement Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assumptions used to determine benefit obligation
Discount rate	3.85%	3.15%
Health care cost trend rate assumed for next year	5.60%	5.70%
Final health care cost trend rate	4.50%	4.50%
Year that final health care trend rate is reached	2038	2038
Average estimated duration of benefit cash flows	6 years 1 month 2 days
Contributions to plan during the year	$ 392	$ 332	$ 300
Expected contribution	$ 400