Employee Benefit Plans - Other Compensation Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Classification of liabilities in the Consolidated Balance Sheets
Current liabilities	$ 967	$ 902
Total liabilities	4,550	3,959
Plan expenses	1,000	800	$ 700
Accrued compensation
Classification of liabilities in the Consolidated Balance Sheets
Current liabilities	967	902
Other long-term liabilities
Classification of liabilities in the Consolidated Balance Sheets
Long-term liabilities	$ 3,583	$ 3,057