Retirement Plans - Non-contributory defined benefit pension plan - (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Components of net periodic pension cost and other amounts recognized in other comprehensive income
Interest cost	$ 270	$ 281
Service cost	19	31
Expected return on plan assets	(582)	(553)
Recognized net actuarial loss	194	188
Net periodic pension expense	$ (99)	$ (53)
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
Defined benefit plan requisite service period under 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