Credit Facility (Details)	1 Months Ended
	Feb. 28, 2018	Sep. 30, 2018 USD ($)	Feb. 18, 2018 USD ($)	Dec. 31, 2017 USD ($)
Credit facility
Amount outstanding		$ 0		$ 0
Remaining borrowing capacity		31,000,000.0
Letters of credit outstanding, amount		$ 4,000,000
Revolving line of credit
Credit facility
Maximum borrowing capacity			$ 35,000,000.0
Increase in maximum borrowing capacity, subject to lender's consent			15,000,000.0
Maximum borrowing capacity, subject to lender's consent			50,000,000.0
Quick ratio minimum	1.50
Maximum consolidated leverage ratio, upper end of range	3.00
Maximum consolidated leverage ratio, lower end of range	2.50
Minimum fixed charge coverage ratio	1.25
Revolving line of credit | Minimum
Credit facility
Unused facility fee (as a percent)	0.00%
Revolving line of credit | Maximum
Credit facility
Unused facility fee (as a percent)	0.20%
Revolving line of credit | Prime rate | Minimum
Credit facility
Variable rate basis spread (as a percent)	(0.25%)
Revolving line of credit | Prime rate | Maximum
Credit facility
Variable rate basis spread (as a percent)	0.50%
Revolving line of credit | LIBOR | Minimum
Credit facility
Variable rate basis spread (as a percent)	1.75%
Revolving line of credit | LIBOR | Maximum
Credit facility
Variable rate basis spread (as a percent)	2.50%
Letters of credit
Credit facility
Maximum borrowing capacity			$ 10,000,000.0