Available-for-Sale Marketable Securities - Schedule of Fair Values and Continuous Unrealized Loss Positions of Available-for-Sale Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 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	$ 48,259	$ 132,755
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(50)	(119)
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Gross Unrealized Losses	0	0
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	48,259	132,755
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(50)	(119)
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	3,177	16,392
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(9)	(1)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	3,177	16,392
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(9)	(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	5,487	5,596
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(11)	(1)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	5,487	5,596
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(11)	(1)
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	27,689	71,322
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(27)	(94)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	27,689	71,322
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(27)	(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	11,906	39,445
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(3)	(23)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	11,906	39,445
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	$ (3)	$ (23)