FAIR VALUE MEASUREMENT - Fair Value Measurements, Recurring and Nonrecurring Basis (Details) - USD ($) $ in Thousands		Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		$ 2,587,305	$ 2,358,597
Derivative assets: Interest rate swaps	[1]	18,537	8,031
Derivative liabilities: Interest rate swaps	[1]	902	56
State and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		1,580,594	802,802
Other Stocks and Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		78,255	10,137
Residential MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[2]	810,010	1,310,642
Commercial MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[2]	118,446	235,016
Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments		6,094	5,965
Total asset fair value measurements		2,611,936	2,374,162
Total liability fair value measurements		40,172	11,742
Fair Value, Recurring [Member] | State and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		1,580,594	802,802
Fair Value, Recurring [Member] | Other Stocks and Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		78,255	10,137
Fair Value, Recurring [Member] | Residential MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[3]	810,010	1,310,642
Fair Value, Recurring [Member] | Commercial MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[3]	118,446	235,016
Fair Value, Recurring [Member] | Interest Rate Swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets: Interest rate swaps		18,537	9,600
Derivative liabilities: Interest rate swaps		40,172	11,742
Fair Value, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments		6,094	5,965
Total asset fair value measurements		6,094	5,965
Total liability fair value measurements		0	0
Fair Value, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | State and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		0	0
Fair Value, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Stocks and Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		0	0
Fair Value, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Residential MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		0	0
Fair Value, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Commercial MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		0	0
Fair Value, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Interest Rate Swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets: Interest rate swaps		0	0
Derivative liabilities: Interest rate swaps		0	0
Fair Value, Recurring [Member] | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments		0	0
Total asset fair value measurements		2,605,842	2,368,197
Total liability fair value measurements		40,172	11,742
Fair Value, Recurring [Member] | Significant Other Observable Inputs (Level 2) | State and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		1,580,594	802,802
Fair Value, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Other Stocks and Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		78,255	10,137
Fair Value, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Residential MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[3]	810,010	1,310,642
Fair Value, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Commercial MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value	[3]	118,446	235,016
Fair Value, Recurring [Member] | Significant Other Observable Inputs (Level 2) | Interest Rate Swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets: Interest rate swaps		18,537	9,600
Derivative liabilities: Interest rate swaps		40,172	11,742
Fair Value, Recurring [Member] | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments		0	0
Total asset fair value measurements		0	0
Total liability fair value measurements		0	0
Fair Value, Recurring [Member] | Significant Unobservable Inputs (Level 3) | State and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		0	0
Fair Value, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Other Stocks and Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		0	0
Fair Value, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Residential MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		0	0
Fair Value, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Commercial MBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale, at estimated fair value		0	0
Fair Value, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Interest Rate Swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets: Interest rate swaps		0	0
Derivative liabilities: Interest rate swaps		0	0
Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreclosed assets		120	472
Impaired loans	[4]		18,586
Total asset fair value measurements		10,773	19,058
Fair Value, Nonrecurring [Member] | Collateral-Dependent Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	[4]	10,653
Fair Value, Nonrecurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreclosed assets		0	0
Impaired loans			0
Total asset fair value measurements		0	0
Fair Value, Nonrecurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Collateral-Dependent Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans		0
Fair Value, Nonrecurring [Member] | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreclosed assets		0	0
Impaired loans			0
Total asset fair value measurements		0	0
Fair Value, Nonrecurring [Member] | Significant Other Observable Inputs (Level 2) | Collateral-Dependent Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans		0
Fair Value, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreclosed assets		120	472
Impaired loans	[4]		18,586
Total asset fair value measurements		10,773	$ 19,058
Fair Value, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) | Collateral-Dependent Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	[4]	$ 10,653

[1] Net derivative assets are included in other assets and net derivative liabilities are included in other liabilities on the consolidated balance sheets. Included in the fair value of net derivative assets and net derivative liabilities are credit valuation adjustments reflecting counterparty credit risk and our credit risk. At December 31, 2020, we had no credit exposure related to interest rate swaps with financial institutions and $18.5 million related to interest rate swaps with customers. At December 31, 2019, our credit exposure related to interest rate swaps with financial institutions and interest rate swaps with customers was $883,000 and $8.0 million, respectively. The credit risk associated with customer transactions is partially mitigated as these are generally secured by the non-cash collateral securing the underlying transaction being hedged.

[2] All MBS issued and/or guaranteed by U.S. government agencies or U.S. GSEs.

[3] All MBS are issued and/or guaranteed by U.S. government agencies or U.S. GSEs.

[4] Consists of individually evaluated loans. Loans for which the fair value of the collateral and commercial real estate fair value of the properties is less than cost basis are presented net of allowance. Losses on these loans represent charge-offs which are netted against the allowance for loan losses.