Investments in debt and equity securities - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Amortized costs	$ 623,614	$ 578,165
Fair values	593,583	589,772
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized costs	32,555	34,739
Fair values	32,525	36,323
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized costs	268,626	249,757
Fair values	255,437	258,102
Foreign
Debt Securities, Available-for-sale [Line Items]
Amortized costs	311,894	287,240
Fair values	295,134	288,883
U.S. Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized costs	10,539	6,429
Fair values	$ 10,487	$ 6,464