Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Due in one year, Amortized Cost	$ 2
Due after one year through five years, Amortized Cost	211
Due after five years through ten years, Amortized Cost	2,231
Due after ten years, Amortized Cost	731,848
Totals, Amortized Cost	734,292
Due in one year, Estimated Fair Value	2
Due after one year through five years, Estimated Fair Value	211
Due after five years through ten years, Estimated Fair Value	2,281
Due after ten years, Estimated Fair Value	728,389
Totals, Estimated Fair Value	730,883
Due in one year, Amortized Cost, Held to Maturity	0
Due after one year through five years, Amortized Cost, Held to Maturity	1,207
Due after five years through ten years, Amortized Cost, Held to Maturity	14,011
Due after ten years, Amortized Cost, Held to Maturity	499,626
Totals, Amortized Cost, Held to Maturity	514,844	$ 602,536
Due in one year, Estimated Fair Value, Held to Maturity	0
Due after one year through five years, Estimated Fair Value, Held to Maturity	1,226
Due after five years through ten years, Estimated Fair Value, Held to Maturity	14,067
Due after ten years, Estimated Fair Value, Held to Maturity	502,872
Estimated Fair Value, Held to Maturity	$ 518,165	$ 603,203