Investments in debt and equity securities - Debt Securities According to Contractual Terms (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Amortized costs
In one year or less	$ 68,394
After one year through five years	299,977
After five years through ten years	186,766
After ten years	30,978
Amortized costs, total	586,115	$ 592,648
Fair values
In one year or less	69,164
After one year through five years	313,410
After five years through ten years	201,134
After ten years	34,187
Fair values, total	$ 617,895	$ 605,721