Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 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	206
Due after five years through ten years, Amortized Cost	2,223
Due after ten years, Amortized Cost	679,264
Totals, Amortized Cost	681,695
Due in one year, Estimated Fair Value	2
Due after one year through five years, Estimated Fair Value	206
Due after five years through ten years, Estimated Fair Value	2,279
Due after ten years, Estimated Fair Value	675,749
Totals, Estimated Fair Value	678,236
Due in one year, Amortized Cost, Held to Maturity	0
Due after one year through five years, Amortized Cost, Held to Maturity	1,199
Due after five years through ten years, Amortized Cost, Held to Maturity	4,098
Due after ten years, Amortized Cost, Held to Maturity	531,270
Totals, Amortized Cost, Held to Maturity	536,567	$ 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	1,233
Due after five years through ten years, Estimated Fair Value, Held to Maturity	4,165
Due after ten years, Estimated Fair Value, Held to Maturity	536,725
Estimated Fair Value, Held to Maturity	$ 542,123	$ 603,203