Summary of Significant Accounting Policies - Fixed Assets (Details) |
12 Months Ended |
---|---|
Dec. 31, 2017 | |
Furniture, Equipment and Technology and Software | Minimum | |
Property, Plant and Equipment [Line Items] | |
Fixed assets, useful life | 3 years |
Furniture, Equipment and Technology and Software | Maximum | |
Property, Plant and Equipment [Line Items] | |
Fixed assets, useful life | 10 years |
Internal-Use Software Development | Minimum | |
Property, Plant and Equipment [Line Items] | |
Fixed assets, useful life | 3 years |
Internal-Use Software Development | Maximum | |
Property, Plant and Equipment [Line Items] | |
Fixed assets, useful life | 10 years |