Retirement Plans - Non-contributory defined benefit pension plan - (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Components of net periodic pension cost and other amounts recognized in other comprehensive income
Interest cost	$ 270	$ 281	$ 810	$ 843
Service cost	19	31	58	95
Expected return on plan assets	(582)	(553)	(1,746)	(1,660)
Recognized net actuarial loss	194	188	581	564
Net periodic pension expense	$ (99)	$ (53)	$ (297)	$ (158)
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	Labor and Related Expense	Labor and Related Expense
Employee hired on or after January 1, 2006 | Maximum
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
Non-contributory defined benefit pension plan | Employees hired on or before December 31, 2005 | Minimum
Retirement Plans
Requisite age of employees for receiving retirement benefits under the plan			21 years