Investments in debt and equity securities - Debt Securities According Contractual Terms (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Amortized costs, In one year or less	$ 60,697
Amortized costs, After one year through five years	196,352
Amortized costs, After five years through ten years	235,408
Amortized costs, After ten years	42,213
Amortized costs, Total	534,670
Fair Values, In one year or less	60,879
Fair Values, After one year through five years	201,272
Fair Values, After five years through ten years	244,617
Fair Values, After ten years	45,099
Debt securities, fair values total	$ 551,867