Available-for-Sale Marketable Securities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Position	Dec. 31, 2016 USD ($) Position
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sale of available-for-sale marketable securities	$ 8,755,000	$ 34,003,000
Realized gains	$ 5,000	$ 23,000
Number of available-for-sale marketable securities in unrealized loss positions | Position	30	18
Total number of positions | Position	31	34
Other-than-temporary impairments on securities	$ 0	$ 0
Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sale of available-for-sale marketable securities	8,755,000	34,003,000
Realized gains	$ 5,000	$ 23,000