Goodwill - Goodwill by reportable segment (Details) - USD ($) $ in Millions |
Dec. 31, 2019 |
Dec. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|---|
| Changes in the carrying amount of goodwill | |||
| Goodwill | $ 21.3 | $ 21.3 | $ 21.3 |
| Curtis Palmer | |||
| Changes in the carrying amount of goodwill | |||
| Goodwill | 14.4 | 14.4 | 14.4 |
| Morris | |||
| Changes in the carrying amount of goodwill | |||
| Goodwill | 3.3 | 3.3 | 3.3 |
| Nipigon | |||
| Changes in the carrying amount of goodwill | |||
| Goodwill | $ 3.6 | $ 3.6 | $ 3.6 |