Summary of Significant Accounting Policies - Property and Equipment and Internal Use Software (Details) - USD ($) |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|
| Property and equipment: | |||
| Depreciation | $ 18,483,928 | $ 16,333,164 | $ 12,403,273 |
| Property and equipment, net | $ 63,061,890 | 57,450,059 | |
| Computer equipment and software | |||
| Property and equipment: | |||
| Property and equipment, useful life | 3 years | ||
| Office equipment | |||
| Property and equipment: | |||
| Property and equipment, useful life | 5 years | ||
| Furniture and fixtures | Minimum | |||
| Property and equipment: | |||
| Property and equipment, useful life | 5 years | ||
| Furniture and fixtures | Maximum | |||
| Property and equipment: | |||
| Property and equipment, useful life | 7 years | ||
| Software development | |||
| Property and equipment: | |||
| Property and equipment, useful life | 3 years | ||
| Depreciation | $ 10,013,198 | 9,031,147 | $ 8,648,096 |
| Property and equipment, net | $ 29,270,620 | $ 20,234,055 | |