Employee Benefit Plans - Components of Net Periodic Benefit Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Components of net periodic benefit cost:
Net periodic income	$ (64)	$ (99)	$ (191)	$ (297)
General and administrative
Components of net periodic benefit cost:
Service cost	16	18	48	53
Interest expense
Components of net periodic benefit cost:
Interest cost	53	57	160	170
Other income
Components of net periodic benefit cost:
Expected long-term return on assets	(51)	(62)	(152)	(187)
Prior service cost amortization	(113)	(114)	(340)	(340)
Actuarial loss amortization	$ 31	$ 2	$ 93	$ 7