Short-Term Investments - Additional Information (Detail) - USD ($) |
3 Months Ended | 9 Months Ended | ||
---|---|---|---|---|
Sep. 30, 2018 |
Sep. 30, 2017 |
Sep. 30, 2018 |
Sep. 30, 2017 |
|
Investments Debt And Equity Securities [Abstract] | ||||
Realized gains (losses) on short-term investments | $ 0 | $ 0 | $ 0 | $ 0 |
Other-than-temporary impairments recognized | $ 0 | $ 0 | $ 0 | $ 0 |