Share-Based Compensation - Schedule of Restricted Shares Activity (Detail) - Restricted Shares [Member] - $ / shares |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | |||
| Number of shares, Outstanding, Beginning balance | 2,087,392 | ||
| Number of shares, Granted | 934,518 | ||
| Number of shares, Vested | (782,802) | ||
| Number of shares, Forfeited | (588,100) | ||
| Number of shares, Outstanding, Ending balance | 1,651,008 | 2,087,392 | |
| Number of shares, Expected to vest | 1,572,618 | ||
| Weighted average grant date fair value, Outstanding, Beginning balance | $ 26.61 | ||
| Weighted average grant date fair value, Granted | 34.63 | $ 22.30 | $ 34.70 |
| Weighted average grant date fair value, Vested | 27.66 | ||
| Weighted average grant date fair value, Forfeited | 26.38 | ||
| Weighted average grant date fair value, Outstanding, Ending balance | 29.95 | $ 26.61 | |
| Weighted average grant date fair value, Expected to vest | $ 29.98 | ||