Available-for-Sale Marketable Securities - Schedule of Fair Values and Continuous Unrealized Loss Positions of Available-for-Sale Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Fair Value	$ 58,152	$ 82,683
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(114)	(70)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	58,152	82,683
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(114)	(70)
Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Fair Value	29,176	39,951
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(87)	(37)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	29,176	39,951
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(87)	(37)
U.S. Government Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Fair Value	3,796	7,794
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(3)	(5)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	3,796	7,794
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(3)	(5)
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Fair Value	8,341	15,907
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(11)	(12)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	8,341	15,907
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(11)	(12)
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Fair Value	16,839	19,031
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(13)	(16)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	16,839	19,031
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	$ (13)	$ (16)