Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Due in one year, Amortized Cost	$ 2,415
Due after one year through five years, Amortized Cost	14,287
Due after five years through ten years, Amortized Cost	44,325
Due after ten years, Amortized Cost	1,071,678
Totals, Amortized Cost	1,132,705
Due in one year, Estimated Fair Value	2,421
Due after one year through five years, Estimated Fair Value	14,636
Due after five years through ten years, Estimated Fair Value	45,235
Due after ten years, Estimated Fair Value	1,071,702
Totals, Estimated Fair Value	1,133,994
Due after one year through five years, Amortized Cost, Held to Maturity	1,254
Due after five years through ten years, Amortized Cost, Held to Maturity	21,922
Due after ten years, Amortized Cost, Held to Maturity	389,348
Totals, Amortized Cost, Held to Maturity	412,524	$ 444,936
Due after one year through five years, Estimated Fair Value, Held to Maturity	1,269
Due after five years through ten years, Estimated Fair Value, Held to Maturity	22,166
Due after ten years, Estimated Fair Value, Held to Maturity	391,841
Estimated Fair Value, Held to Maturity	$ 415,276	$ 437,370