Available-for-Sale Marketable Securities - Schedule of Fair Values and Continuous Unrealized Loss Positions of Available-for-Sale Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 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	$ 86,143	$ 82,683
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(60)	(70)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	86,143	82,683
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(60)	(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	68,138	39,951
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(57)	(37)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	68,138	39,951
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(57)	(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		7,794
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses		(5)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value		7,794
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses		(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	5,334	15,907
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(1)	(12)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	5,334	15,907
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(1)	(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	12,672	19,031
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(2)	(16)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	12,672	19,031
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	$ (2)	$ (16)