Benefit Plans - Defined Benefit Plans - Components of Pension Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Supplemental Retirement Plan
Components of net periodic benefit cost:
Service cost	$ 62	$ 81	$ 124	$ 162
Interest cost	237	259	474	518
Amortization of net actuarial loss	73	69	146	138
Net periodic benefit cost	372	409	744	818
Split-Dollar Life Insurance Benefit Plan
Components of net periodic benefit cost:
Interest cost	56	60	113	121	$ 243
Amortization of prior transition obligation	(16)	(17)	(32)	(35)
Net periodic benefit cost	$ 40	$ 43	$ 81	$ 86