Investments - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Cash and cash equivalents	$ 19,906,000	$ 62,510,000	$ 24,607,000	$ 14,482,000
Investments	28,798,000	30,053,000
Held-to-Maturity Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Investments	$ 28,800,000	30,100,000
Investment maturity	One year or less
Money Market Funds [Member] | Level 1 [Member]
Schedule of Held-to-maturity Securities [Line Items]
Cash and cash equivalents	$ 17,000	$ 55,600,000