Investments in debt and equity securities available-for-sale - Debt Securities According to Contractual Terms (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Amortized costs
In one year or less	$ 45,924
After one year through five years	357,596
After five years through ten years	206,149
After ten years	40,753
Amortized costs, total	650,422	$ 665,646
Fair values
In one year or less	45,914
After one year through five years	356,396
After five years through ten years	202,527
After ten years	40,931
Fair values, total	$ 645,768	$ 671,441