Significant Accounting Policies - Intangible Assets (Details) |
12 Months Ended |
|---|---|
Dec. 31, 2018 | |
| Purchased software | Minimum | |
| Finite-Lived Intangible Assets [Line Items] | |
| Useful life (in years) | 2 years |
| Purchased software | Maximum | |
| Finite-Lived Intangible Assets [Line Items] | |
| Useful life (in years) | 3 years |
| Other intangible assets | Minimum | |
| Finite-Lived Intangible Assets [Line Items] | |
| Useful life (in years) | 2 years |
| Other intangible assets | Maximum | |
| Finite-Lived Intangible Assets [Line Items] | |
| Useful life (in years) | 3 years |
| Software development costs | Minimum | |
| Finite-Lived Intangible Assets [Line Items] | |
| Useful life (in years) | 2 years |
| Software development costs | Maximum | |
| Finite-Lived Intangible Assets [Line Items] | |
| Useful life (in years) | 3 years |