Borrowings - Additional Information (Details) |
Sep. 30, 2018 |
Dec. 31, 2017 |
Sep. 30, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|---|---|---|---|---|---|
| Debt Instrument [Line Items] | |||||
| Annual weighted average interest rate | 7.11% | 6.87% | 6.98% | ||
| Three months ended | |||||
| Debt Instrument [Line Items] | |||||
| Annual weighted average interest rate | 7.82% | 7.19% | |||
| Nine months ended | |||||
| Debt Instrument [Line Items] | |||||
| Annual weighted average interest rate | 7.75% | 6.96% |