Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Due in one year, Amortized Cost	$ 1,896
Due after one year through five years, Amortized Cost	4,014
Due after five years through ten years, Amortized Cost	31,848
Due after ten years, Amortized Cost	183,313
Totals, Amortized Cost	221,071
Due in one year, Estimated Fair Value	1,906
Due after one year through five years, Estimated Fair Value	4,202
Due after five years through ten years, Estimated Fair Value	33,332
Due after ten years, Estimated Fair Value	185,686
Totals, Estimated Fair Value	225,126
Due in one year, Amortized Cost, Held to Maturity	0
Due after one year through five years, Amortized Cost, Held to Maturity	0
Due after five years through ten years, Amortized Cost, Held to Maturity	1,125
Due after ten years, Amortized Cost, Held to Maturity	801,357
Totals, Amortized Cost, Held to Maturity	802,482	676,426
Due in one year, Estimated Fair Value, Held to Maturity	0
Due after one year through five years, Estimated Fair Value, Held to Maturity	0
Due after five years through ten years, Estimated Fair Value, Held to Maturity	1,134
Due after ten years, Estimated Fair Value, Held to Maturity	816,659
Estimated Fair Value, Held to Maturity	$ 817,793	$ 688,779