Investments in debt and equity securities - Debt Securities According to Contractual Terms (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Amortized costs
In one year or less	$ 55,856
After one year through five years	359,043
After five years through ten years	190,383
After ten years	40,621
Amortized costs, total	645,903	$ 665,646
Fair values
In one year or less	55,917
After one year through five years	355,605
After five years through ten years	186,494
After ten years	39,867
Fair values, total	$ 637,883	$ 671,441