Investments in debt and equity securities - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized costs	$ 634,616	$ 646,728
Fair values	608,314	611,934
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized costs	26,619	30,104
Fair values	26,487	29,835
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized costs	259,084	272,362
Fair values	244,479	254,316
Foreign
Debt Securities, Available-for-sale [Line Items]
Amortized costs	318,408	315,184
Fair values	306,885	299,137
U.S. Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized costs	30,505	29,078
Fair values	$ 30,463	$ 28,646