Investment Securities - Contractual Maturities of Investment Securities Available for Sale (Detail) $ in Thousands	Sep. 30, 2015 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Due in one year, Amortized Cost	$ 600
Due after one year through five years, Amortized Cost	70,286
Due after five years through ten years, Amortized Cost	71,953
Due after ten years, Amortized Cost	66,221
Total investment securities available for sale, Amortized Cost	799,264
Due in one year, Fair Value	602
Due after one year through five years, Fair Value	71,237
Due after five years through ten years, Fair Value	70,566
Due after ten years, Fair Value	64,655
Total investment securities available for sale, Fair Value	797,389
Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Securities without a single maturity date, Amortized Cost	569,182
Securities without a single maturity date, Fair Value	569,650
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Securities without a single maturity date, Amortized Cost	21,022
Securities without a single maturity date, Fair Value	$ 20,679