Investments in debt and equity securities - Debt Securities According to Contractual Terms (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Amortized costs
In one year or less	$ 58,344
After one year through five years	311,487
After five years through ten years	194,553
After ten years	30,860
Amortized costs, total	595,244	$ 599,412
Fair values
In one year or less	59,145
After one year through five years	321,937
After five years through ten years	201,439
After ten years	32,924
Fair values, total	$ 615,445	$ 631,386