Investments in debt and equity securities - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized costs	$ 631,294	$ 646,728
Fair values	610,236	611,934
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized costs	22,201	30,104
Fair values	22,031	29,835
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized costs	242,656	272,362
Fair values	231,474	254,316
Foreign
Debt Securities, Available-for-sale [Line Items]
Amortized costs	332,723	315,184
Fair values	323,391	299,137
U.S. Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized costs	33,714	29,078
Fair values	$ 33,340	$ 28,646