Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Due in one year, Amortized Cost	$ 2,607
Due after one year through five years, Amortized Cost	14,986
Due after five years through ten years, Amortized Cost	46,263
Due after ten years, Amortized Cost	913,687
Debt Securities, Available-for-sale, Amortized Cost	977,543	$ 1,136,010
Due in one year, Estimated Fair Value	2,613
Due after one year through five years, Estimated Fair Value	15,401
Due after five years through ten years, Estimated Fair Value	47,273
Due after ten years, Estimated Fair Value	918,793
Debt Securities, Available-for-sale	984,080	1,115,036
Due within one year, Amortized Cost, Held to Maturity	1,263
Due after one year through five years, Amortized Cost, Held to Maturity	0
Due after five years through ten years, Amortized Cost, Held to Maturity	20,747
Due after ten years, Amortized Cost, Held to Maturity	371,439
Totals, Amortized Cost, Held to Maturity	393,449	444,936
Due within one year, Estimated Fair Value, Held to Maturity	1,274
Due after one year through five years, Estimated Fair Value, Held to Maturity	0
Due after five years through ten years, Estimated Fair Value, Held to Maturity	21,035
Due after ten years, Estimated Fair Value, Held to Maturity	377,390
Debt Securities Held to Maturity, Estimated Fair Value	$ 399,699	$ 437,370