Long-Term Debt - Carrying Amount of Notes (Details) - Senior Notes - USD ($) |
Dec. 31, 2017 |
Dec. 31, 2016 |
May 05, 2015 |
|---|---|---|---|
| Debt Instrument [Line Items] | |||
| Convertible senior notes, principal amount | $ 230,000,000 | $ 230,000,000 | $ 230,000,000 |
| Unamortized debt discount | (15,930,370) | (22,070,838) | |
| Unamortized debt issuance costs | (3,150,695) | (4,365,151) | |
| Convertible senior notes, net | $ 210,918,935 | $ 203,564,011 |