Investment Securities - Summary of Available-for-Sale Debt Securities by Stated Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015
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	15,000
Amortized Cost, Due after ten years	0
Mortgage-backed securities, Amortized Cost	355,904
Total debt securities, Amortized Cost	370,904
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	15,172
Fair Value, Due after ten years	0
Mortgage-backed securities, Fair Value	358,717
Total debt securities, Fair Value	$ 373,889