Acquisitions and divestments - 2017 Divestments (Details) - USD ($) $ in Millions |
1 Months Ended | 3 Months Ended | 12 Months Ended |
|---|---|---|---|
Nov. 30, 2017 |
Jun. 30, 2017 |
Dec. 31, 2017 |
|
| Disposal Group, Disposed of by Sale, Not Discontinued Operations | Selkirk Cogen Partners, LP | |||
| Divestments | |||
| Sale of ownership percentage | 17.70% | ||
| Proceeds from disposal | $ 1.0 | ||
| Equity investment after impairment write down | $ 0.0 | ||
| Disposal Group, Disposed of by Sale, Not Discontinued Operations | Selkirk Cogen Partners, LP | Earnings from unconsolidated affiliates | |||
| Divestments | |||
| Gain on sale of equity method investment | $ 1.0 | ||
| Selkirk Cogen Partners, LP | Earnings from unconsolidated affiliates | |||
| Divestments | |||
| Impairment charge on pending sale | $ 10.6 | 10.6 | |
| Gain on sale of equity method investment | $ 1.0 | ||
| Selkirk Cogen Partners, LP | Disposal Group, Disposed of by Sale, Not Discontinued Operations | |||
| Divestments | |||
| Sale of ownership percentage | 17.70% |