Subsequent event - Additional Information (Detail) - USD ($) $ in Millions |
Feb. 28, 2018 |
Aug. 31, 2017 |
|---|---|---|
| Subsequent Event [Line Items] | ||
| Amount of repurchased ordinary shares | $ 30.0 | |
| Subsequent Event | ||
| Subsequent Event [Line Items] | ||
| Additional amount of repurchased ordinary shares | $ 30.0 | |
| Amount of repurchased ordinary shares | $ 60.0 |