Investments in debt and equity securities - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized costs	$ 620,806	$ 631,294
Fair values	597,057	610,236
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized costs	18,725	22,201
Fair values	18,499	22,031
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized costs	240,573	242,656
Fair values	228,668	231,474
Foreign
Debt Securities, Available-for-sale [Line Items]
Amortized costs	326,560	332,723
Fair values	315,661	323,391
U.S. Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized costs	34,948	33,714
Fair values	$ 34,229	$ 33,340