Derivative financial instruments, Breakdown of fair value amounts (Details) - USD ($) $ in Thousands |
12 Months Ended | |
|---|---|---|
Dec. 31, 2017 |
Dec. 31, 2016 |
|
| Currency Swap [Member] | ||
| Breakdown of fair value amount of derivative financial instruments [Abstract] | ||
| Term of agreement | 5 years | |
| Interest Rate Derivatives [Member] | Cash Flow Hedge [Member] | ||
| Breakdown of fair value amount of derivative financial instruments [Abstract] | ||
| Assets | $ 8,230 | $ 3,822 |
| Liabilities | $ 329,731 | $ 349,266 |