Investments in debt and equity securities - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Amortized costs	$ 624,272	$ 578,165
Fair values	582,461	589,772
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized costs	30,759	34,739
Fair values	30,119	36,323
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized costs	279,392	249,757
Fair values	257,575	258,102
Foreign
Debt Securities, Available-for-sale [Line Items]
Amortized costs	297,490	287,240
Fair values	278,511	288,883
U.S. Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized costs	16,631	6,429
Fair values	$ 16,256	$ 6,464