Investments in debt and equity securities - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized costs	$ 595,244	$ 599,412
Fair values	615,445	631,386
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized costs	42,015	45,138
Fair values	44,089	47,603
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized costs	276,870	285,962
Fair values	289,159	305,450
Foreign
Debt Securities, Available-for-sale [Line Items]
Amortized costs	269,792	261,748
Fair values	275,585	271,711
U.S. Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized costs	6,567	6,564
Fair values	$ 6,612	$ 6,622