Short-Term Investments - Summary of Short-Term Investments (Detail) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 66,919,000
Gross Unrealized Losses	(22,000)
Fair Value	66,897,000	$ 0
U.S Government-Sponsored Enterprise Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	53,384,000
Gross Unrealized Losses	(20,000)
Fair Value	53,364,000
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13,535,000
Gross Unrealized Losses	(2,000)
Fair Value	$ 13,533,000