Investments in debt and equity securities - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized costs	$ 628,153	$ 646,728
Fair values	585,406	611,934
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized costs	23,941	30,104
Fair values	23,278	29,835
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized costs	250,552	272,362
Fair values	231,038	254,316
Foreign
Debt Securities, Available-for-sale [Line Items]
Amortized costs	321,046	315,184
Fair values	299,748	299,137
U.S. Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized costs	32,614	29,078
Fair values	$ 31,342	$ 28,646