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Fair value measurements - Changes in Level 3 Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2023
Mar. 31, 2022
ARS purchase commitments    
Liabilities    
Liabilities Beginning Balance $ 0 $ 0
Total Realized and Unrealized Losses 0 0
Purchases and Issuances 0 0
Sales and Settlements 0 0
Transfers In (Out) 0 0
Liabilities Ending Balance 0 0
Auction rate securities    
Assets    
Assets Beginning Balance 31,776 31,804
Total Realized and Unrealized Losses 0 0
Purchases and Issuances 0 0
Sales and Settlements 0 0
Transfers In (Out) 0 0
Assets Ending Balance $ 31,776 $ 31,804