Investments in debt and equity securities - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized costs	$ 635,247	$ 646,728
Fair values	601,927	611,934
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized costs	26,273	30,104
Fair values	25,949	29,835
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized costs	249,888	272,362
Fair values	233,200	254,316
Foreign
Debt Securities, Available-for-sale [Line Items]
Amortized costs	326,937	315,184
Fair values	311,354	299,137
U.S. Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized costs	32,149	29,078
Fair values	$ 31,424	$ 28,646