Cash Equivalents and Investments - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Cash And Cash Equivalents [Abstract]
Investment in equity with a fair value and unrealized gain	$ 900,000		$ 900,000
Investments in continuous unrealized loss position for 12 months or longer	0		0
Interest income	$ 1,300,000	$ 800,000	$ 3,300,000	$ 1,800,000