Debt - (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Line of Credit Facility, Initiation Date	Oct. 30, 2015
Long-term debt	$ 4,435,000	$ 4,725,000
Maximum amount outstanding	6,000,000	4,700,000
Average amount outstanding	$ 2,200,000	$ 1,900,000
Weighted-average interest rate	3.00%	2.90%
Commitment fees	$ 100,000	$ 100,000
Interest expense	$ 100,000	$ 100,000
Debt covenants, Maximum leverage ratio	1.10%
Debt covenants, Minimum tangible net worth	$ 5,000,000
Debt covenants, Maximum capital expenditures	$ 5,000,000
Fixed charge coverage ratio	5.625
Total available borrowings	$ 28,900,000
Debt covenants, EBITDA	6,800,000
Capital expenditures	$ 2,500,000
Federal Funds Effective Swap Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.50%
Minimum [Member] | London Interbank Offered Rate (LIBOR)
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.50%
Minimum [Member] | Adjusted London Interbank Offered Rate L I B O R
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.50%
Maximum [Member] | London Interbank Offered Rate (LIBOR)
Line of Credit Facility [Line Items]
Basis spread on variable rate	2.00%
Maximum [Member] | Adjusted London Interbank Offered Rate L I B O R
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.00%
Credit and Security Agreement [Member]
Line of Credit Facility [Line Items]
Revolving credit facility, term	3 years
Line of Credit Facility, Maximum Borrowing Capacity	$ 45,000,000
Line of credit facility, expiration date	2020-12
Swing Line Loans [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 10,000,000
Letters of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 10,000,000