Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Significant Accounting Policies [Line Items]
Maximum maturity period of highly liquid investments	3 months
Allowances for doubtful accounts	$ 680,000	$ 527,000
Accruals for premium deficiency	0	0
Fair value, assets, level 1 to level 2 transfers	0	0
Fair value, assets, level 2 to level 1 transfers	0	0
Fair value, liabilities, level 1 to level 2 transfers	0	0
Fair value, liabilities, level 2 to level 1 transfers	0	$ 0
Accounting Standards Update 2016-09 Compensation - Stock Compensation [Member]
Schedule Of Significant Accounting Policies [Line Items]
Excess tax benefit	$ 5,800,000
Website Development [Member]
Schedule Of Significant Accounting Policies [Line Items]
Useful life of property and equipment	3 years
Software [Member]
Schedule Of Significant Accounting Policies [Line Items]
Useful life of property and equipment	3 years
Minimum [Member]
Schedule Of Significant Accounting Policies [Line Items]
Short-term investment maturity period	3 months
Useful life of property and equipment	3 years
Maximum [Member]
Schedule Of Significant Accounting Policies [Line Items]
Useful life of property and equipment	27 years 6 months