Fair Value of Financial Assets - Additional Information (Detail) - USD ($) |
9 Months Ended | 12 Months Ended |
---|---|---|
Sep. 30, 2018 |
Dec. 31, 2017 |
|
Fair Value Disclosures [Abstract] | ||
Available-for-sale investments, maximum contractual maturity period | 1 year | |
Fair value, assets transfers from Level 1 to Level 2 | $ 0 | $ 0 |
Fair value, assets transfers from Level 2 to Level 1 | 0 | 0 |
Fair Value, assets transfers into (out of) Level 3 | $ 0 | $ 0 |