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, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Fair Value	$ 4,756	$ 132,755
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(6)	(119)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	4,756	132,755
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(6)	(119)
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	4,756	71,322
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(6)	(94)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	4,756	71,322
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	$ (6)	(94)
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		39,445
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses		(23)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value		39,445
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses		(23)
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		16,392
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, Fair Value		16,392
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses		(1)
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		5,596
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, Fair Value		5,596
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses		$ (1)