Investments in debt and equity securities - Debt Securities According Contractual Terms (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Amortized costs
In one year or less	$ 79,668
After one year through five years	304,610
After five years through ten years	184,215
After ten years	30,919
Amortized costs	599,412	$ 592,648
Fair values
In one year or less	80,400
After one year through five years	317,567
After five years through ten years	198,443
After ten years	34,976
Fair values	$ 631,386	$ 605,721