Significant Accounting Policies - Property and Equipment (Details) |
12 Months Ended |
|---|---|
Dec. 31, 2017 | |
| Computer equipment | |
| Property, Plant and Equipment [Line Items] | |
| Useful life (in years) | 3 years |
| Office furniture and equipment | |
| Property, Plant and Equipment [Line Items] | |
| Useful life (in years) | 4 years |
| Leasehold improvements | Minimum | |
| Property, Plant and Equipment [Line Items] | |
| Useful life (in years) | 3 years |
| Leasehold improvements | Maximum | |
| Property, Plant and Equipment [Line Items] | |
| Useful life (in years) | 13 years |