Employee Benefit Plans - Components of Net Periodic Benefit Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Components of net periodic benefit cost:
Net periodic income	$ (64)	$ (99)	$ (127)	$ (198)
General and administrative
Components of net periodic benefit cost:
Service cost	16	18	32	35
Interest expense
Components of net periodic benefit cost:
Interest cost	53	57	107	113
Other income
Components of net periodic benefit cost:
Expected long-term return on assets	(51)	(62)	(101)	(124)
Prior service cost amortization	(113)	(114)	(227)	(227)
Actuarial loss amortization	$ 31	$ 2	$ 62	$ 5