Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Due in one year, Amortized Cost	$ 2,410
Due after one year through five years, Amortized Cost	10,625
Due after five years through ten years, Amortized Cost	17,381
Due after ten years, Amortized Cost	1,105,594
Totals, Amortized Cost	1,136,010
Due in one year, Estimated Fair Value	2,413
Due after one year through five years, Estimated Fair Value	10,794
Due after five years through ten years, Estimated Fair Value	17,622
Due after ten years, Estimated Fair Value	1,084,207
Totals, Estimated Fair Value	1,115,036
Due in one year, Amortized Cost, Held to Maturity	0
Due after one year through five years, Amortized Cost, Held to Maturity	1,238
Due after five years through ten years, Amortized Cost, Held to Maturity	25,039
Due after ten years, Amortized Cost, Held to Maturity	418,659
Totals, Amortized Cost, Held to Maturity	444,936	$ 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	1,250
Due after five years through ten years, Estimated Fair Value, Held to Maturity	24,744
Due after ten years, Estimated Fair Value, Held to Maturity	411,376
Estimated Fair Value, Held to Maturity	$ 437,370	$ 518,165