Retirement Plans - Other (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) employee item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Projected benefit obligation:
Balance, beginning of the year	$ 3,027	$ 2,865
Service cost	80	109	$ 112
Interest cost	85	104	102
Actuarial (gain) loss	(469)	(45)
Benefits paid to participants	(481)	(6)
Balance, end of year	2,242	3,027	2,865
Funded status of the plan
Excess of benefit obligation over the value of plan assets	(2,242)	(3,027)	(2,865)
Unrecognized net actuarial loss	347	818	905
Unrecognized prior service cost	44	52	69
Accrued benefit cost	(1,851)	(2,157)	(1,891)
Net amounts recognized in the consolidated balance sheets
Accrued benefit cost	(2,242)	(3,027)
Accumulated other comprehensive loss	391	870
Accrued benefit cost	(1,851)	$ (2,157)	(1,891)
Benefits expected to be paid in each of the next five years and thereafter
2018	242
2019	109
2020	91
2021	182
2022	235
2023 through 2026	582
Total	$ 1,441
Weighted-average rates used
Discount rate on the benefit obligation (as a percent)	3.35%	3.78%
Rate of employee compensation increase (as a percent)		4.00%
Pension expense
Service cost	$ 80	$ 109	112
Interest cost	85	104	102
Amortization of prior service cost	8	17	17
Net loss amortization	1	42	51
Net periodic benefit cost	$ 174	$ 272	$ 282
Maximum
Weighted-average rates used
Rate of employee compensation increase (as a percent)	2.50%
Minimum
Weighted-average rates used
Rate of employee compensation increase (as a percent)	1.75%
WAPA
Number of months to be in service for applicability of retirement plan	3 months
Number of weeks per year of service for one-time lump sum payment at retirement	14 days
Maximum number of retirees per year under the plan | employee	5
Number of retirees per year under the retirement plan | item	144
WAPA | Maximum
Number of weeks per year of service for one-time lump sum payment at retirement	315 days