Securities - Amortized Cost and Fair Value of Debt Securities Available for Sale Securities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-Sale Securities [Line Items]
Due in one year or less, Amortized Cost	$ 5,693
Due from one year to five years, Amortized Cost	45,110
Due from five to ten years, Amortized Cost	63,043
Due after ten years, Amortized Cost	49,792
Amortized Cost	533,219	488,838
Due in one year or less, Estimated Fair Value	5,757
Due from one year to five years, Estimated Fair Value	46,340
Due from five to ten years, Estimated Fair Value	64,201
Due after ten years, Estimated Fair Value	50,795
Mortgage-backed securities, Estimated Fair Value	374,712
Total Fair Value	541,805	486,235
Mortgage-Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Mortgage-backed securities, Amortized cost	369,581
Amortized Cost	369,581	349,441
Total Fair Value	$ 374,712	$ 347,750