Employee Benefits - Components of Net Periodic Pension Cost and Other Changes in Plan Assets and Benefit (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Components of Net Periodic Pension Cost
Service cost—benefits earned during the period	$ 0	$ 0
Interest cost on projected benefit obligation	967	1,025
Expected return on plan assets	(1,567)	(1,582)
Amortization of prior service credit	544	474
Recognized net actuarial loss	0	0
Net Periodic Pension Expense	$ (56)	$ (83)