Fair Value - Fair Value Assets Measured on a Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets
Short-term investments	$ 70,671	$ 63,651
U.S. Treasury Securities [Member]
Assets
Short-term investments	70,671	63,651
Fair Value Measurements, Recurring [Member]
Assets
Total assets	81,267	80,810
Fair Value Measurements, Recurring [Member] | Money Market Funds [Member]
Assets
Money market funds	10,596	17,159
Fair Value Measurements, Recurring [Member] | U.S. Treasury Securities [Member]
Assets
Short-term investments	70,671	63,651
Level 1 [Member] | Fair Value Measurements, Recurring [Member]
Assets
Total assets	81,267	80,810
Level 1 [Member] | Fair Value Measurements, Recurring [Member] | Money Market Funds [Member]
Assets
Money market funds	10,596	17,159
Level 1 [Member] | Fair Value Measurements, Recurring [Member] | U.S. Treasury Securities [Member]
Assets
Short-term investments	70,671	63,651
Level 2 [Member] | Fair Value Measurements, Recurring [Member]
Assets
Total assets	0	0
Level 2 [Member] | Fair Value Measurements, Recurring [Member] | Money Market Funds [Member]
Assets
Money market funds	0	0
Level 2 [Member] | Fair Value Measurements, Recurring [Member] | U.S. Treasury Securities [Member]
Assets
Short-term investments	0	0
Level 3 [Member] | Fair Value Measurements, Recurring [Member]
Assets
Total assets	0	0
Level 3 [Member] | Fair Value Measurements, Recurring [Member] | Money Market Funds [Member]
Assets
Money market funds	0	0
Level 3 [Member] | Fair Value Measurements, Recurring [Member] | U.S. Treasury Securities [Member]
Assets
Short-term investments	$ 0	$ 0