Investments in debt and equity securities - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Amortized costs	$ 646,728	$ 578,165
Fair values	611,934	589,772
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized costs	30,104	34,739
Fair values	29,835	36,323
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized costs	272,362	249,757
Fair values	254,316	258,102
Foreign
Debt Securities, Available-for-sale [Line Items]
Amortized costs	315,184	287,240
Fair values	299,137	288,883
U.S. Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized costs	29,078	6,429
Fair values	$ 28,646	$ 6,464