Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Due in one year, Amortized Cost	$ 1
Due after one year through five years, Amortized Cost	12,246
Due after five years through ten years, Amortized Cost	16,676
Due after ten years, Amortized Cost	484,651
Totals, Amortized Cost	513,574
Due in one year, Estimated Fair Value	1
Due after one year through five years, Estimated Fair Value	12,748
Due after five years through ten years, Estimated Fair Value	17,557
Due after ten years, Estimated Fair Value	498,711
Totals, Estimated Fair Value	529,017
Due in one year, Amortized Cost, Held to Maturity	0
Due after one year through five years, Amortized Cost, Held to Maturity	1,161
Due after five years through ten years, Amortized Cost, Held to Maturity	845
Due after ten years, Amortized Cost, Held to Maturity	672,406
Totals, Amortized Cost, Held to Maturity	674,412	$ 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	1,201
Due after five years through ten years, Estimated Fair Value, Held to Maturity	936
Due after ten years, Estimated Fair Value, Held to Maturity	697,462
Estimated Fair Value, Held to Maturity	$ 699,599	$ 732,209