Available-for-Sale Marketable Securities - Schedule of Fair Values and Continuous Unrealized Loss Positions of Available-for-Sale Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Fair Value	$ 82,683	$ 17,668
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(70)	(9)
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Fair Value		0
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Gross Unrealized Losses		0
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	82,683	17,668
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(70)	(9)
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	39,951
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(37)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	39,951
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(37)
US Government Corporations and Agencies Securities [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	15,907	9,919
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(12)	(6)
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Fair Value		0
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Gross Unrealized Losses		0
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	15,907	9,919
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(12)	(6)
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	19,031	5,227
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(16)	(2)
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Fair Value		0
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Gross Unrealized Losses		0
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	19,031	5,227
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	$ (16)	(2)
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Fair Value		2,522
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses		(1)
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Fair Value		0
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Gross Unrealized Losses		0
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value		2,522
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses		$ (1)