Employee Benefit Plans - Components of the Net Periodic Pension and Postretirement Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Pension Plans
Components of net period pension plans benefit
Service cost	$ 395	$ 357
Interest cost	4,019	5,233
Expected return on assets	(6,722)	(6,124)
Amortization of actuarial loss	52	52
Partial curtailment	(306)	0
Net periodic pension plans benefit	(2,562)	(482)
Other Postretirement Benefit Plan
Components of net period pension plans benefit
Service cost	52	67
Interest cost	40	56
Amortization of prior service credit	(2)	(2)
Amortization of actuarial loss	49	42
Net periodic pension plans benefit	$ 139	$ 163