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Cash, cash equivalents, short-term investments and restricted cash - Short-term Investments and Restricted Cash and Investments (Details) - USD ($)
$ in Thousands
Mar. 31, 2020
Dec. 31, 2019
Mar. 31, 2019
Dec. 31, 2018
Cash and Cash Equivalents [Abstract]        
Cash and cash equivalents $ 90,905 $ 70,467    
Restricted cash 10,932 10,972    
Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 101,837 $ 81,439 $ 71,370 $ 69,238