Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Due in one year, Amortized Cost	$ 0
Due after one year through five years, Amortized Cost	10,578
Due after five years through ten years, Amortized Cost	15,637
Due after ten years, Amortized Cost	470,609
Totals, Amortized Cost	496,824
Due in one year, Estimated Fair Value	0
Due after one year through five years, Estimated Fair Value	10,969
Due after five years through ten years, Estimated Fair Value	16,397
Due after ten years, Estimated Fair Value	482,843
Totals, Estimated Fair Value	510,209
Due in one year, Amortized Cost, Held to Maturity	0
Due after one year through five years, Amortized Cost, Held to Maturity	1,169
Due after five years through ten years, Amortized Cost, Held to Maturity	853
Due after ten years, Amortized Cost, Held to Maturity	639,127
Totals, Amortized Cost, Held to Maturity	641,149	$ 726,530
Due in one year, Estimated Fair Value, Held to Maturity	0
Due after one year through five years, Estimated Fair Value, Held to Maturity	1,203
Due after five years through ten years, Estimated Fair Value, Held to Maturity	943
Due after ten years, Estimated Fair Value, Held to Maturity	661,635
Estimated Fair Value, Held to Maturity	$ 663,781	$ 732,209