Investments in debt and equity securities - Debt Securities According Contractual Terms (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Amortized costs
In one year or less	$ 35,974
After one year through five years	358,738
After five years through ten years	221,792
After ten years	49,142
Amortized costs, Total	665,646
Fair values
In one year or less	36,092
After one year through five years	362,206
After five years through ten years	221,994
After ten years	51,149
Fair values, Total	$ 671,441