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Short-term investments (Details) - USD ($)
Sep. 30, 2021
Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]    
Amortized cost   $ 14,997,000
Gross unrealized gains   1,000
Fair value   14,998,000
Short-term investments $ 0 14,998,000
U.S. Treasury bonds    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost   14,997,000
Gross unrealized gains   1,000
Fair value   $ 14,998,000