Cash Equivalents and Short-Term Investments - Summary of Cost and Estimated Fair Value of Available-for-Sale Securities Based on Stated Effective Maturities (Detail) $ in Thousands	Sep. 30, 2017 USD ($)
Investments Debt And Equity Securities [Abstract]
Amortized Cost Basis, Mature within one year	$ 151,769
Amortized Cost Basis, Mature after one year to three years	28,128
Amortized Cost Basis, Total cash equivalents and short-term investments	179,897
Estimated Fair Value, Mature within one year	151,725
Estimated Fair Value, Mature after one year to three years	28,070
Estimated Fair Value, Total cash equivalents and short-term investments	$ 179,795