Available-for-Sale Marketable Securities - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) Position	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) Position
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sale of available-for-sale marketable securities	$ 10,850,000	$ 5,030,000
Realized loss	$ 15,000	$ (3,000)
Number of available-for-sale marketable securities in unrealized loss positions | Position	22		30
Total number of positions | Position	27		31
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	10,850,000
Realized loss	$ 15,000