Investments in debt and equity securities - Debt Securities According to Contractual Terms (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Amortized costs
In one year or less	$ 48,839
After one year through five years	364,717
After five years through ten years	182,578
After ten years	38,899
Amortized costs, total	635,033	$ 665,646
Fair values
In one year or less	48,844
After one year through five years	359,727
After five years through ten years	177,919
After ten years	37,932
Fair values, total	$ 624,422	$ 671,441