Investment Securities - Amortized Cost and Estimated Fair Values of Investments Securities (Detail) - USD ($)	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020
Debt Securities Available for Sale
Amortized Cost	$ 1,833,330,000	$ 1,395,081,000
Gross Unrealized Gains	20,860,000	25,225,000
Gross Unrealized Losses	(6,622,000)	(6,042,000)
Allowance for Credit Losses	0	0	$ 0
Estimated Fair Value	1,847,568,000	1,414,264,000
Debt Securities Held to Maturity
Amortized Cost	235,778,000	284,563,000
Gross Unrealized Gains	11,241,000	14,163,000
Gross Unrealized Losses	0	0
Allowance for Credit Losses	0	0	$ 0
Estimated Fair Value	247,019,000	298,726,000
Obligations of U.S. government agencies
Debt Securities Available for Sale
Amortized Cost	1,126,342,000	795,555,000
Gross Unrealized Gains	11,628,000	17,710,000
Gross Unrealized Losses	(4,554,000)	(891,000)
Allowance for Credit Losses	0	0
Estimated Fair Value	1,133,416,000	812,374,000
Debt Securities Held to Maturity
Amortized Cost	226,925,000	273,667,000
Gross Unrealized Gains	10,926,000	13,774,000
Gross Unrealized Losses	0	0
Allowance for Credit Losses	0	0
Estimated Fair Value	237,851,000	287,441,000
Obligations of states and political subdivisions
Debt Securities Available for Sale
Amortized Cost	141,168,000	123,347,000
Gross Unrealized Gains	6,208,000	5,748,000
Gross Unrealized Losses	(452,000)	0
Allowance for Credit Losses	0	0
Estimated Fair Value	146,924,000	129,095,000
Debt Securities Held to Maturity
Amortized Cost	8,853,000	10,896,000
Gross Unrealized Gains	315,000	389,000
Gross Unrealized Losses	0	0
Allowance for Credit Losses	0	0
Estimated Fair Value	9,168,000	11,285,000
Corporate bonds
Debt Securities Available for Sale
Amortized Cost	2,473,000	2,459,000
Gross Unrealized Gains	83,000	85,000
Gross Unrealized Losses	0	0
Allowance for Credit Losses	0	0
Estimated Fair Value	2,556,000	2,544,000
Asset backed securities
Debt Securities Available for Sale
Amortized Cost	563,347,000	473,720,000
Gross Unrealized Gains	2,941,000	1,682,000
Gross Unrealized Losses	(1,616,000)	(5,151,000)
Allowance for Credit Losses	0	0
Estimated Fair Value	$ 564,672,000	$ 470,251,000

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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