Schedule of Share-Based Compensation Arrangements by Share-Based Payment Award (Detail) - USD ($) |
3 Months Ended | 9 Months Ended | ||
|---|---|---|---|---|
Sep. 30, 2016 |
Sep. 30, 2015 |
Sep. 30, 2016 |
Sep. 30, 2015 |
|
| Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] | ||||
| Weighted-average grant date fair value per share | $ 17.09 | $ 56.22 | $ 16.07 | $ 52.98 |
| Intrinsic value of options exercised | $ 930,417 | $ 5,307,365 | $ 1,555,342 | $ 15,361,206 |
| Cash received from stock option exercises | $ 687,332 | $ 1,953,938 | $ 1,094,855 | $ 4,534,288 |