Investments in debt and equity securities - Debt Securities According to Contractual Terms (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Amortized costs
In one year or less	$ 77,995
After one year through five years	303,007
After five years through ten years	160,301
After ten years	33,145
Amortized costs, total	574,448	$ 592,648
Fair values
In one year or less	78,047
After one year through five years	307,426
After five years through ten years	166,107
After ten years	32,573
Fair values, total	$ 584,153	$ 605,721