Investments in Marketable Securities - Schedule of Fair Value of Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Marketable Securities [Line Items]
Marketable securities	$ 60,478	$ 79,538
Level 1 [Member]
Marketable Securities [Line Items]
Marketable securities	10,988	17,026
Level 2 [Member]
Marketable Securities [Line Items]
Marketable securities	49,490	62,512
U.S. Treasury Securities [Member]
Marketable Securities [Line Items]
Marketable securities	10,988	17,026
U.S. Treasury Securities [Member] | Level 1 [Member]
Marketable Securities [Line Items]
Marketable securities	10,988	17,026
Commercial Paper [Member]
Marketable Securities [Line Items]
Marketable securities	35,552	48,624
Commercial Paper [Member] | Level 2 [Member]
Marketable Securities [Line Items]
Marketable securities	35,552	48,624
Asset Backed Securities [Member]
Marketable Securities [Line Items]
Marketable securities	13,938	13,888
Asset Backed Securities [Member] | Level 2 [Member]
Marketable Securities [Line Items]
Marketable securities	$ 13,938	$ 13,888