v3.20.2

Equity compensation plans (Tables) | 9 Months Ended
--- | ---
Sep. 30, 2020
Equity compensation plans |
Schedule of changes in outstanding LTIP notional units | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | Grant Date |
​ | ​ | ​ | ​ | ​ | ​ | ​ | Weighted-Average |
​ |  | Notional Shares | ​ | ​ | ​ | ​ | Fair Value per Notional Share |
Outstanding at December 31, 2019 |  | 3,578,092 | ​ | ​ | ​ | $ | 2.38 | ​
Granted |  | 1,866,748 | ​ | ​ | ​ | ​ | 2.49 | ​
Vested and redeemed |  | (1,702,571) | ​ | ​ | ​ | ​ | 2.34 | ​
Forfeitures |  | (31,929) | ​ | ​ | ​ | ​ | 2.42 | ​
Outstanding at September 30, 2020 |  | 3,710,340 | ​ | ​ | ​ | $ | 2.45 | ​

X
- References No definition available. + Details
Name: | us-gaap_DisclosureOfCompensationRelatedCostsSharebasedPaymentsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tabular disclosure of activity for award under share-based payment arrangement. Includes, but is not limited to, outstanding award at beginning and end of year, granted, exercised, forfeited, and weighted-average grant date fair value. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
Name: | us-gaap_ScheduleOfShareBasedCompensationActivityTableTextBlock
Namespace Prefix: | us-gaap_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration