Investments in debt and equity securities available-for-sale - Debt Securities According Contractual Terms (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Amortized costs, In one year or less	$ 41,517
Amortized costs, After one year through five years	231,495
Amortized costs, After five years through ten years	218,310
Amortized costs, After ten years	31,795
Amortized costs, Total	523,117
Fair Values, In one year or less	41,649
Fair Values, After one year through five years	238,946
Fair Values, After five years through ten years	229,754
Fair Values, After ten years	35,011
Debt securities, fair values total	$ 545,360