Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Due in one year, Amortized Cost	$ 2
Due after one year through five years, Amortized Cost	235
Due after five years through ten years, Amortized Cost	3,324
Due after ten years, Amortized Cost	751,309
Totals, Amortized Cost	754,870
Due in one year, Estimated Fair Value	2
Due after one year through five years, Estimated Fair Value	236
Due after five years through ten years, Estimated Fair Value	3,452
Due after ten years, Estimated Fair Value	732,205
Totals, Estimated Fair Value	735,895
Due in one year, Amortized Cost, Held to Maturity	0
Due after one year through five years, Amortized Cost, Held to Maturity	1,215
Due after five years through ten years, Amortized Cost, Held to Maturity	20,631
Due after ten years, Amortized Cost, Held to Maturity	474,189
Totals, Amortized Cost, Held to Maturity	496,035	$ 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	1,228
Due after five years through ten years, Estimated Fair Value, Held to Maturity	20,291
Due after ten years, Estimated Fair Value, Held to Maturity	467,120
Estimated Fair Value, Held to Maturity	$ 488,639	$ 518,165