Cash and cash equivalents (Details) - USD ($) $ in Thousands |
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
Dec. 31, 2014 |
|---|---|---|---|---|
| Cash and cash equivalents [abstract] | ||||
| Cash and cash equivalents | $ 669,387 | $ 594,811 | $ 514,712 | $ 354,154 |
| Cash held to satisfy the requirements of non-recourse debt agreements | 263,000 | |||
| U.S. Dollar [Member] | ||||
| Cash and cash equivalents [abstract] | ||||
| Cash and cash equivalents | 319,400 | 343,954 | ||
| Euro [Member] | ||||
| Cash and cash equivalents [abstract] | ||||
| Cash and cash equivalents | 288,625 | 196,382 | ||
| Algerian Dinar [Member] | ||||
| Cash and cash equivalents [abstract] | ||||
| Cash and cash equivalents | 13,628 | 10,736 | ||
| South African Rand [Member] | ||||
| Cash and cash equivalents [abstract] | ||||
| Cash and cash equivalents | 40,999 | 39,689 | ||
| Others [Member] | ||||
| Cash and cash equivalents [abstract] | ||||
| Cash and cash equivalents | $ 6,735 | $ 4,050 |