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Investment securities (Details) - USD ($)
$ in Thousands
Mar. 31, 2024
Dec. 31, 2023
Current investments:    
Amortized cost $ 312,621 $ 376,532
Gross unrealized holding gains 12 139
Estimated fair value 312,633 376,671
Government debt securities (held-to-maturity)    
Current investments:    
Amortized cost 312,621 376,532
Gross unrealized holding gains 12 139
Estimated fair value $ 312,633 $ 376,671