Investments in debt and equity securities - Debt Securities According Contractual Terms (Detail) $ in Thousands	Dec. 31, 2016 USD ($)
Amortized costs
In one year or less	$ 53,313
After one year through five years	227,877
After five years through ten years	225,206
After ten years	82,783
Amortized costs, Total	589,179
Fair values
In one year or less	53,644
After one year through five years	232,594
After five years through ten years	225,483
After ten years	83,398
Debt securities, fair values total	$ 595,119