Fair Value Disclosures - Schedule of Short-Term Investments (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Short-term Investments, Amortized Cost	$ 21,939	$ 32,914
Short-term Investments, Gross Unrealized Holding Gains	0	0
Short-term Investments, Gross Unrealized Holding Losses	(23)	(7)
Short-term Investments, Fair Value	$ 21,916	$ 32,907
Available-for-sale securities, maturity period	1 year	1 year
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Short-term Investments, Amortized Cost	$ 21,939	$ 32,914
Short-term Investments, Gross Unrealized Holding Gains	0	0
Short-term Investments, Gross Unrealized Holding Losses	(23)	(7)
Short-term Investments, Fair Value	$ 21,916	$ 32,907