Line of Credit - Additional Information (Detail)	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Maximum ratio of total liabilities to tangible net worth		2
Trailing period used for calculating ratios		12 months
Line of Credit
Debt Instrument [Line Items]
Total availability under line of credit agreement	$ 10,000,000.0
Line of credit borrowings	0	$ 0
Letters of credit outstanding amount	2,700,000
Available borrowing under letter of credit	$ 7,300,000
Company's leverage ratio		0.002
Line of Credit | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread plus (minus) on variable interest rate	1.25%
Line of Credit | Prime Rate
Debt Instrument [Line Items]
Basis spread plus (minus) on variable interest rate	(0.50%)