Retirement Plans - Amounts Expected to be Recognized as Components of Net Periodic Pension Costs (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Pension plan
Amounts expected to be recognized as component of net periodic pension costs during the next fiscal year:
Amortization net actuarial gain or (loss)	$ 64
Amortization prior service cost	0
SERP
Amounts expected to be recognized as component of net periodic pension costs during the next fiscal year:
Amortization net actuarial gain or (loss)	$ 63