Investments in debt and equity securities available-for-sale - Debt Securities According to Contractual Terms (Details) $ in Thousands	Sep. 30, 2017 USD ($)
Amortized costs
In one year or less	$ 31,735
After one year through five years	304,284
After five years through ten years	233,228
After ten years	65,246
Amortized costs, total	634,493
Fair values
In one year or less	31,933
After one year through five years	309,497
After five years through ten years	234,022
After ten years	67,951
Fair values, total	$ 643,403