Subsequent events (Details) - Subsequent Events [Member] - USD ($) $ / shares in Units, $ in Millions |
May 11, 2018 |
May 10, 2018 |
|---|---|---|
| Disclosure of non-adjusting events after reporting period [line items] | ||
| Dividend approved (in dollars per share) | $ 0.32 | |
| Dividend approved expected date to be paid | Jun. 15, 2018 | |
| New Revolving Credit Facility [Member] | ||
| Disclosure of non-adjusting events after reporting period [line items] | ||
| Principal amount | $ 215 | |
| Maximum borrowings capacity of debt | $ 300 | |
| Maturity date | December 31, 2021 | |
| New Revolving Credit Facility [Member] | Top of range [member] | ||
| Disclosure of non-adjusting events after reporting period [line items] | ||
| Additional borrowings capacity of debt | $ 85 |