Investment Securities - Amortized Cost and Estimated Fair Values of Investments Securities (Detail) - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Debt Securities Available for Sale
Amortized Cost	$ 1,129,534,000	$ 946,751,000
Gross Unrealized Gains	23,270,000	10,721,000
Gross Unrealized Losses	(9,847,000)	(7,334,000)
Allowance for credit losses	0	0
Estimated Fair Value	1,142,957,000	950,138,000
Debt Securities Held to Maturity
Amortized Cost	310,696,000	375,606,000
Gross Unrealized Gains	14,762,000	6,399,000
Gross Unrealized Losses	0	(480,000)
Estimated Fair Value	325,458,000	381,525,000
Allowance for Credit Losses	0	0
Obligations of U.S. government agencies
Debt Securities Available for Sale
Amortized Cost	556,295,000	466,139,000
Gross Unrealized Gains	17,821,000	7,261,000
Gross Unrealized Losses	(228,000)	(420,000)
Allowance for credit losses	0
Estimated Fair Value	573,888,000	472,980,000
Debt Securities Held to Maturity
Amortized Cost	299,797,000	361,785,000
Gross Unrealized Gains	14,363,000	6,072,000
Gross Unrealized Losses	0	(480,000)
Estimated Fair Value	314,160,000	367,377,000
Allowance for Credit Losses	0
Obligations of states and political subdivisions
Debt Securities Available for Sale
Amortized Cost	107,840,000	106,373,000
Gross Unrealized Gains	4,898,000	3,229,000
Gross Unrealized Losses	(13,000)	(1,000)
Allowance for credit losses	0
Estimated Fair Value	112,725,000	109,601,000
Debt Securities Held to Maturity
Amortized Cost	10,899,000	13,821,000
Gross Unrealized Gains	399,000	327,000
Gross Unrealized Losses	0	0
Estimated Fair Value	11,298,000	14,148,000
Allowance for Credit Losses	0
Corporate bonds
Debt Securities Available for Sale
Amortized Cost	2,451,000	2,430,000
Gross Unrealized Gains	101,000	102,000
Gross Unrealized Losses	0	0
Allowance for credit losses	0
Estimated Fair Value	2,552,000	2,532,000
Asset backed securities
Debt Securities Available for Sale
Amortized Cost	462,948,000	371,809,000
Gross Unrealized Gains	450,000	129,000
Gross Unrealized Losses	(9,606,000)	(6,913,000)
Allowance for credit losses	0
Estimated Fair Value	$ 453,792,000	$ 365,025,000

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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