Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Amortized Cost
Amortized cost, due in one year, available for sale	$ 0
Amortized cost, due after one year through five years, available for sale	138,993
Amortized cost, due after five years through ten years, available for sale	142,939
Amortized cost, due after ten years, available for sale	847,602
Amortized Cost	1,129,534	$ 946,751
Estimated Fair Value
Estimated fair value, due in one year, available for sale	0
Estimated fair value, due after one year through five years, available for sale	139,469
Estimated fair value, due after five years through ten years, available for sale	143,492
Estimated fair value, due after ten years, available for sale	859,996
Estimated Fair Value	1,142,957	950,138
Amortized Cost
Amortized cost, due within one year, held to maturity	0
Amortized cost, due after one year through five years, held to maturity	0
Amortized cost, due after five years through ten years, held to maturity	20,439
Amortized cost, due after ten years, held to maturity	290,257
Amortized cost, held to maturity	310,696	375,606
Estimated Fair Value
Estimated fair value, due within one year, held to maturity	0
Estimated fair value, due after one year through five years, held to maturity	0
Estimated fair value, due after five years through ten years, held to maturity	21,485
Estimated fair value, due after ten years, held to maturity	303,973
Estimated fair value, held to maturity	$ 325,458	$ 381,525