Fair Value Measurements - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets:
Total assets	$ 100,758	$ 36,944
Money Market Funds
Assets:
Total assets	5,028	13,804
Short-term Investments - Commercial Paper
Assets:
Total assets	45,064	14,982
Short-term Investments - Corporate Notes
Assets:
Total assets	18,126	2,824
Short-term investments - U.S. Government Bonds
Assets:
Total assets	32,141	5,006
Investment
Assets:
Total assets	399	328
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Total assets	5,427	14,132
Quoted Prices in Active Markets for Identical Assets (Level 1) | Money Market Funds
Assets:
Total assets	5,028	13,804
Quoted Prices in Active Markets for Identical Assets (Level 1) | Investment
Assets:
Total assets	399	328
Significant Other Observable Inputs (Level 2)
Assets:
Total assets	95,331	22,812
Significant Other Observable Inputs (Level 2) | Short-term Investments - Commercial Paper
Assets:
Total assets	45,064	14,982
Significant Other Observable Inputs (Level 2) | Short-term Investments - Corporate Notes
Assets:
Total assets	18,126	2,824
Significant Other Observable Inputs (Level 2) | Short-term investments - U.S. Government Bonds
Assets:
Total assets	$ 32,141	$ 5,006