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Restricted cash (Details) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Jun. 30, 2018
Dec. 31, 2017
Cash and Cash Equivalents [Abstract]        
Cash and cash equivalents $ 782,884 $ 1,204,018 $ 1,598,502  
Restricted cash 187,401 211,017 259,971  
Total cash, cash equivalents and restricted cash $ 970,285 $ 1,415,035 $ 1,858,473 $ 2,024,125