Investments in debt and equity securities - Debt Securities According Contractual Terms (Detail) $ in Thousands	Dec. 31, 2015 USD ($)
Investments, Debt and Equity Securities [Abstract]
Amortized costs, In one year or less	$ 27,118
Amortized costs, After one year through five years	226,497
Amortized costs, After five years through ten years	230,161
Amortized costs, After ten years	54,111
Amortized costs, Total	537,887
Fair Values, In one year or less	27,256
Fair Values, After one year through five years	230,842
Fair Values, After five years through ten years	235,219
Fair Values, After ten years	55,279
Debt securities, fair values total	$ 548,596