Investments in debt and equity securities - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Amortized costs	$ 603,211	$ 578,165
Fair values	589,396	589,772
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized costs	32,934	34,739
Fair values	33,182	36,323
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized costs	252,113	249,757
Fair values	248,050	258,102
Foreign
Debt Securities, Available-for-sale [Line Items]
Amortized costs	311,239	287,240
Fair values	301,284	288,883
U.S. Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized costs	6,925	6,429
Fair values	$ 6,880	$ 6,464