Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Due in one year, Amortized Cost	$ 2,435
Due after one year through five years, Amortized Cost	12,486
Due after five years through ten years, Amortized Cost	17,764
Due after ten years, Amortized Cost	1,056,326
Totals, Amortized Cost	1,089,011
Due in one year, Estimated Fair Value	2,434
Due after one year through five years, Estimated Fair Value	12,501
Due after five years through ten years, Estimated Fair Value	17,740
Due after ten years, Estimated Fair Value	1,023,285
Totals, Estimated Fair Value	1,055,960
Due in one year, Amortized Cost, Held to Maturity	0
Due after one year through five years, Amortized Cost, Held to Maturity	1,231
Due after five years through ten years, Amortized Cost, Held to Maturity	25,955
Due after ten years, Amortized Cost, Held to Maturity	432,711
Totals, Amortized Cost, Held to Maturity	459,897	$ 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	1,240
Due after five years through ten years, Estimated Fair Value, Held to Maturity	25,210
Due after ten years, Estimated Fair Value, Held to Maturity	419,837
Estimated Fair Value, Held to Maturity	$ 446,287	$ 518,165