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Commitments and contingencies - Custodian facility (Details)
$ in Thousands, SFr in Millions
Mar. 31, 2020
USD ($)
Mar. 31, 2020
CHF (SFr)
Dec. 31, 2019
USD ($)
Short-term Debt [Line Items]      
Revolving credit agreement borrowings $ 576,486   $ 484,287
Custodian bank facility      
Short-term Debt [Line Items]      
Revolving credit agreement borrowings 75,900   0
Cash and investments held on deposit with custodian 108,200   $ 0
EUR denominated borrowings | Custodian bank facility      
Short-term Debt [Line Items]      
Revolving credit agreement borrowings | SFr   SFr 0.6  
USD denominated borrowings equivalent | Custodian bank facility      
Short-term Debt [Line Items]      
Revolving credit agreement borrowings $ 700