Credit Facility - February 2015 Amended Credit Facility (Details)	1 Months Ended
	Feb. 28, 2015 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Letter of Credit
Line of Credit Facility [Line Items]
Revolving credit facility	$ 10,000,000
Third Amended Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility	35,000,000
Increase in revolving credit facility (up to)	15,000,000.0
Aggregate maximum of revolving credit facility	$ 50,000,000
Adjusted quick ratio (at least)	1.5
Maximum consolidated leverage ratio, upper end of the range	3.00
Maximum consolidated leverage ratio, lower end of the range	2.50
Fixed charge coverage ratio (at least)	1.25
Outstanding Balance on Credit Facility		$ 0	$ 0
Amount available under the credit facility		31,000,000	30,700,000
Letters of credit outstanding under the subfacility		$ 4,000,000	$ 4,300,000
Third Amended Credit Facility | Minimum
Line of Credit Facility [Line Items]
Unused revolving line facility fee (as a percent)	0.00%
Third Amended Credit Facility | Maximum
Line of Credit Facility [Line Items]
Unused revolving line facility fee (as a percent)	0.20%
Third Amended Credit Facility | Prime Rate | Minimum
Line of Credit Facility [Line Items]
Interest rate	(0.25%)
Third Amended Credit Facility | Prime Rate | Maximum
Line of Credit Facility [Line Items]
Interest rate	0.50%
Third Amended Credit Facility | London Interbank Offered Rate (LIBOR) | Minimum
Line of Credit Facility [Line Items]
Interest rate	1.75%
Third Amended Credit Facility | London Interbank Offered Rate (LIBOR) | Maximum
Line of Credit Facility [Line Items]
Interest rate	2.50%