Investments in debt and equity securities - Debt Securities According to Contractual Terms (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Amortized costs
In one year or less	$ 64,866
After one year through five years	363,235
After five years through ten years	140,784
After ten years	30,657
Amortized costs	599,542	$ 599,412
Fair values
In one year or less	65,678
After one year through five years	371,531
After five years through ten years	146,686
After ten years	33,383
Fair values	$ 617,278	$ 631,386