Investments in debt and equity securities - Debt Securities According Contractual Terms (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Amortized costs
In one year or less	$ 68,377
After one year through five years	336,402
After five years through ten years	157,147
After ten years	30,722
Amortized costs	592,648	$ 608,686
Fair values
In one year or less	68,517
After one year through five years	341,773
After five years through ten years	162,778
After ten years	32,653
Fair values	$ 605,721	$ 602,020