Employee Retirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension plan
Components of net periodic cost:
Service cost	$ 102	$ 89
Interest cost	389	403
Expected return on plan assets	(475)	(448)
Amortization of net (gain) loss	176	181
Net periodic pension cost	192	225
Employer contributions during the period	14	529
Pension plan | Minimum
Components of net periodic cost:
Estimated total required contributions	72
Other postretirement benefit cost
Components of net periodic cost:
Service cost	47	51
Interest cost	58	70
Amortization of net (gain) loss	(52)	(27)
Net periodic pension cost	$ 53	$ 94