INVESTMENTS - Amortized Cost and Estimated Fair Values of Investments Available-for-sale (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Total available-for-sale debt securities	$ 1,419,020	$ 1,310,159
Gross Unrealized Gains	18,867	38,922
Gross Unrealized Losses	(8,332)	(1,060)
Total available-for-sale debt securities	1,429,555	1,348,021
U.S. treasuries and government agencies
Debt Securities, Available-for-sale [Line Items]
Total available-for-sale debt securities	47,477	42,750
Gross Unrealized Gains	260	549
Gross Unrealized Losses	(8)	(2)
Total available-for-sale debt securities	47,729	43,297
State and municipal
Debt Securities, Available-for-sale [Line Items]
Total available-for-sale debt securities	325,384	377,108
Gross Unrealized Gains	5,486	13,470
Gross Unrealized Losses	(3,077)	(211)
Total available-for-sale debt securities	327,793	390,367
Mortgage-backed and asset-backed
Debt Securities, Available-for-sale [Line Items]
Total available-for-sale debt securities	1,039,159	881,201
Gross Unrealized Gains	12,712	24,078
Gross Unrealized Losses	(5,247)	(847)
Total available-for-sale debt securities	1,046,624	904,432
Corporate debt
Debt Securities, Available-for-sale [Line Items]
Total available-for-sale debt securities	7,000	9,100
Gross Unrealized Gains	409	825
Gross Unrealized Losses	0	0
Total available-for-sale debt securities	$ 7,409	$ 9,925

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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Amount, before tax, of unrealized loss 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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Amortized cost of 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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Amount of 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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