Employee Benefit Plans - Accumulated Post-Retirement Benefit Obligation, Funded Status, and Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Change in benefit obligation
Benefit obligation at beginning of year	$ 1,874	$ 1,870
Interest cost	19	19
Benefits paid	(28)	(32)
Benefit obligation at end of period	1,865	1,857
Funded status
Benefit obligation at end of period	(1,874)	(1,870)	$ (1,865)	$ (1,857)
Unamortized loss			186	102
Accrued benefit cost at end of period			$ (1,679)	$ (1,755)
Components of net periodic benefit cost
Interest cost	19	19
Net periodic benefit cost	$ 19	$ 19