Deferred Compensation Benefits - Schedule of Accrued Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
ESBP [Member]
Anticipated Benefit Payments
Current liabilities	$ 659	$ 651
Non-current liabilities	$ 5,750	$ 6,149
Benefit obligation	3.06%	3.41%
Net periodic pension cost	3.41%	3.51%
NDBP [Member]
Anticipated Benefit Payments
Non-current liabilities	$ 5,495	$ 6,366
Benefit obligation	1.80%	1.30%
Net periodic pension cost	1.30%	2.00%
Expected return on plan assets	4.00%	4.00%
BPP [Member]
Anticipated Benefit Payments
Non-current assets	$ 96	$ 84
Benefit obligation	1.62%	1.65%
Net periodic pension cost	1.65%	1.65%
Salary increase rate	1.85%	1.85%
Expected return on plan assets	3.25%	3.25%