Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Due in one year, Amortized Cost	$ 1
Due after one year through five years, Amortized Cost	233
Due after five years through ten years, Amortized Cost	3,229
Due after ten years, Amortized Cost	775,395
Totals, Amortized Cost	778,858
Due in one year, Estimated Fair Value	1
Due after one year through five years, Estimated Fair Value	234
Due after five years through ten years, Estimated Fair Value	3,331
Due after ten years, Estimated Fair Value	750,641
Totals, Estimated Fair Value	754,207
Due in one year, Amortized Cost, Held to Maturity	0
Due after one year through five years, Amortized Cost, Held to Maturity	1,223
Due after five years through ten years, Amortized Cost, Held to Maturity	27,400
Due after ten years, Amortized Cost, Held to Maturity	449,122
Totals, Amortized Cost, Held to Maturity	477,745	$ 514,844
Due in one year, Estimated Fair Value, Held to Maturity	0
Due after one year through five years, Estimated Fair Value, Held to Maturity	1,239
Due after five years through ten years, Estimated Fair Value, Held to Maturity	26,800
Due after ten years, Estimated Fair Value, Held to Maturity	440,582
Estimated Fair Value, Held to Maturity	$ 468,621	$ 518,165