Cash and cash equivalents (Details) - USD ($) $ in Thousands |
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|---|---|---|---|---|
| Cash and cash equivalents [abstract] | ||||
| Cash and cash equivalents | $ 631,542 | $ 669,387 | $ 594,811 | $ 514,712 |
| Cash held to satisfy the requirements of non-recourse debt agreements | 296,000 | |||
| U.S. Dollar [Member] | ||||
| Cash and cash equivalents [abstract] | ||||
| Cash and cash equivalents | 328,716 | 319,400 | ||
| Euro [Member] | ||||
| Cash and cash equivalents [abstract] | ||||
| Cash and cash equivalents | 228,036 | 288,625 | ||
| Algerian Dinar [Member] | ||||
| Cash and cash equivalents [abstract] | ||||
| Cash and cash equivalents | 11,602 | 13,628 | ||
| South African Rand [Member] | ||||
| Cash and cash equivalents [abstract] | ||||
| Cash and cash equivalents | 55,257 | 40,999 | ||
| Others [Member] | ||||
| Cash and cash equivalents [abstract] | ||||
| Cash and cash equivalents | $ 7,931 | $ 6,735 |