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Fair value estimates - Transfers Between Levels (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2020
Dec. 31, 2019
Fair value of financial instruments    
Fair value of assets transfers from level 1 to level 2 $ 0 $ 0
Fair value of assets transfers from level 2 to level 1 0 0
Fair value of liabilities transfers from level 1 to level 2 0 0
Fair value of liabilities transfers from level 2 to level 1 0 0
Fair value of assets transfers into level 3 0 0
Fair value of assets transfers out of level 3 0 0
Fair value of liabilities transfers into level 3 0 0
Fair value of liabilities transfers out of level 3 $ 0 $ 0