Summary of Significant Accounting Policies - Significant Estimates Used by Management in Determining Fair Values of Customer Relationships Acquired (Details) |
3 Months Ended | 12 Months Ended |
|---|---|---|
Mar. 31, 2019 |
Dec. 31, 2018 |
|
| Indefinite-lived Intangible Assets [Line Items] | ||
| Discount rates | 21.00% | |
| Customer attrition rates | 27.00% | |
| Minimum | ||
| Indefinite-lived Intangible Assets [Line Items] | ||
| Discount rates | 20.00% | |
| Customer attrition rates | 20.00% | |
| Maximum | ||
| Indefinite-lived Intangible Assets [Line Items] | ||
| Discount rates | 27.00% | |
| Customer attrition rates | 33.00% |