Short-Term Marketable Securities - Summary of Short-Term Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 35,687	$ 30,570
Unrealized Gain	0	0
Unrealized Loss	29	18
Market Value	35,658	30,552
U.S. treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	34,462	23,295
Unrealized Gain	0	0
Unrealized Loss	29	18
Market Value	34,433	23,277
Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,225	7,275
Unrealized Gain	0	0
Unrealized Loss	0	0
Market Value	$ 1,225	$ 7,275