Investment Securities - Amortized Cost and Fair Value of Debt Securities Available for Sale Securities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
Schedule of Available-for-Sale Securities [Line Items]
Due in one year or less, Amortized Cost	$ 10,647
Due from one year to five years, Amortized Cost	35,432
Due from five to ten years, Amortized Cost	66,554
Due after ten years, Amortized Cost	47,760
Amortized Cost	524,792	488,838	314,720
Due in one year or less, Estimated Fair Value	10,844
Due from one year to five years, Estimated Fair Value	36,856
Due from five to ten years, Estimated Fair Value	67,094
Due after ten years, Estimated Fair Value	48,266
Total Fair Value	529,509	486,235	312,248
Mortgage-Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Mortgage-backed securities, Amortized cost	364,399
Amortized Cost	364,399	349,441	176,818
Mortgage-backed securities, Estimated Fair Value	366,449
Total Fair Value	$ 366,449	$ 347,750	$ 175,654