Investments in debt and equity securities - Debt Securities According to Contractual Terms (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Amortized costs
In one year or less	$ 50,679
After one year through five years	330,611
After five years through ten years	155,433
After ten years	31,292
Amortized costs, total	568,015	$ 608,686
Fair values
In one year or less	50,634
After one year through five years	335,702
After five years through ten years	160,440
After ten years	33,047
Fair values, total	$ 579,823	$ 602,020