Investments in debt and equity securities - Debt Securities According Contractual Terms (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Amortized costs
In one year or less	$ 46,280
After one year through five years	345,026
After five years through ten years	184,655
After ten years	32,725
Amortized costs, Total	608,686	$ 665,646
Fair values
In one year or less	46,271
After one year through five years	341,976
After five years through ten years	181,621
After ten years	32,152
Fair values, Total	$ 602,020	$ 671,441