Investments in debt and equity securities - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized costs	$ 614,213	$ 599,412
Fair values	635,162	631,386
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized costs	40,466	45,138
Fair values	42,462	47,603
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized costs	291,413	285,962
Fair values	305,229	305,450
Foreign
Debt Securities, Available-for-sale [Line Items]
Amortized costs	275,764	261,748
Fair values	280,861	271,711
U.S. Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized costs	6,570	6,564
Fair values	$ 6,610	$ 6,622