Investment Securities - Summary of Available-for-Sale Debt Securities by Stated Maturity (Detail) $ in Thousands	Sep. 30, 2015 USD ($)
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 0
Amortized Cost, Due after one year through five years	0
Amortized Cost, Due after five years through ten years	28,000
Amortized Cost, Due after ten years	7,000
Mortgage-backed securities, Amortized Cost	364,292
Total debt securities, Amortized Cost	399,292
Fair Value, Due in one year or less	0
Fair Value, Due after one year through five years	0
Fair Value, Due after five years through ten years	28,403
Fair Value, Due after ten years	6,919
Mortgage-backed securities, Fair Value	365,760
Total debt securities, Fair Value	$ 401,082