Retirement Plans - Non-contributory defined benefit pension plan (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Components of net periodic pension cost and other amounts recognized in other comprehensive income
Interest cost	$ 289	$ 270
Service cost	30	19
Expected return on plan assets	(590)	(582)
Recognized net actuarial loss	121	194
Net periodic pension expense	$ (150)	$ (99)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible List]	Labor and Related Expense	Labor and Related Expense
Non-contributory defined benefit pension plan | Employees hired on or before December 31, 2005
Retirement Plans
Requisite age of employees for receiving retirement benefits under the plan	21 years
Non-contributory defined benefit pension plan | Employee hired on or after January 1, 2006
Retirement Plans
Requisite age of employees for receiving retirement benefits under the new benefit formula	45 years
Requisite age of employees for receiving retirement benefits under the new benefit formula	5 years