Financial Instruments - Summary of Money Market Funds and Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 37,053	$ 32,876
Unrealized Gain	1
Unrealized Loss	(8)
Estimated Fair Value	37,046	32,876
Money market funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	61	502
Estimated Fair Value	61	502
Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	150	150
Estimated Fair Value	150	150
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	36,842	32,224
Unrealized Gain	1
Unrealized Loss	(8)
Estimated Fair Value	36,835	32,224
Cash and cash equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	35,908	30,055
Unrealized Loss	(8)
Estimated Fair Value	35,900	30,055
Short-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	995	2,671
Unrealized Gain	1
Estimated Fair Value	996	2,671
Long-term restricted investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	150	150
Estimated Fair Value	$ 150	$ 150