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Fair value measurements (Level 3 Rollforward) (Details) - Recurring Fair Value Measurements - Level 3 - Derivative - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Level 3 Rollforward    
Beginning balance $ 30 $ (295)
Realized and unrealized gains (losses) included in derivative (losses) gains 1,009 (1,101)
Settlements (received) paid (805) 1,426
Ending balance 234 30
Gains relating to instruments still held at the reporting date included in derivative (losses) gains for the period $ 234 $ 30