FAIR VALUE - Financial Assets and Liabilities Accounted for at Fair Value On a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	$ 3,791,391
Equity securities	469	$ 445
U.S. Treasury and agency securities
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	2,756,258	2,501,382
Obligations of state and political subdivisions
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	450,171	475,660
Government agency mortgage-backed securities
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	544,986	522,638
Corporate debt securities
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	3,082	4,635
Government agency collateralized mortgage obligations
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	36,894	43,577
Significant Unobservable Inputs, Level 3 [Member] | Commitments to originate real estate loans for sale
Available-for-sale investment securities [Abstract]
Derivative asset	34	14
Recurring [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	3,791,391	3,547,892
Equity securities	469	445
Mortgage loans held for sale	292	1,622
Total	3,792,555	3,550,473
Recurring [Member] | Commitments to originate real estate loans for sale
Available-for-sale investment securities [Abstract]
Derivative asset	34	14
Recurring [Member] | Forward sales commitments
Available-for-sale investment securities [Abstract]
Derivative asset	50
Derivative liability		2
Recurring [Member] | Interest rate swap agreements
Available-for-sale investment securities [Abstract]
Derivative asset	707	1,572
Derivative liability	(388)	(1,074)
Recurring [Member] | U.S. Treasury and agency securities
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	2,756,258	2,501,382
Recurring [Member] | Obligations of state and political subdivisions
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	450,171	475,660
Recurring [Member] | Government agency mortgage-backed securities
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	544,986	522,638
Recurring [Member] | Corporate debt securities
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	3,082	4,635
Recurring [Member] | Government agency collateralized mortgage obligations
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	36,894	43,577
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	2,632,967	2,359,912
Equity securities	469	445
Mortgage loans held for sale	0	0
Total	2,633,436	2,360,357
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | Commitments to originate real estate loans for sale
Available-for-sale investment securities [Abstract]
Derivative asset	0	0
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | Forward sales commitments
Available-for-sale investment securities [Abstract]
Derivative asset	0
Derivative liability		0
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | Interest rate swap agreements
Available-for-sale investment securities [Abstract]
Derivative asset	0	0
Derivative liability	0	0
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | U.S. Treasury and agency securities
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	2,632,967	2,359,912
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | Obligations of state and political subdivisions
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0	0
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | Government agency mortgage-backed securities
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0	0
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | Corporate debt securities
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0	0
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | Government agency collateralized mortgage obligations
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0	0
Recurring [Member] | Significant Observable Inputs, Level 2 [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	1,158,424	1,187,980
Equity securities	0	0
Mortgage loans held for sale	292	1,622
Total	1,159,085	1,190,102
Mortgage loans held for sale	300	1,600
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Commitments to originate real estate loans for sale
Available-for-sale investment securities [Abstract]
Derivative asset	0	0
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Forward sales commitments
Available-for-sale investment securities [Abstract]
Derivative asset	50
Derivative liability		2
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Interest rate swap agreements
Available-for-sale investment securities [Abstract]
Derivative asset	707	1,572
Derivative liability	(388)	(1,074)
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | U.S. Treasury and agency securities
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	123,291	141,470
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Obligations of state and political subdivisions
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	450,171	475,660
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Government agency mortgage-backed securities
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	544,986	522,638
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Corporate debt securities
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	3,082	4,635
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Government agency collateralized mortgage obligations
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	36,894	43,577
Recurring [Member] | Significant Unobservable Inputs, Level 3 [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0	0
Equity securities	0	0
Mortgage loans held for sale	0	0
Total	34	14
Recurring [Member] | Significant Unobservable Inputs, Level 3 [Member] | Commitments to originate real estate loans for sale
Available-for-sale investment securities [Abstract]
Derivative asset	34	14
Recurring [Member] | Significant Unobservable Inputs, Level 3 [Member] | Forward sales commitments
Available-for-sale investment securities [Abstract]
Derivative asset	0
Derivative liability		0
Recurring [Member] | Significant Unobservable Inputs, Level 3 [Member] | Interest rate swap agreements
Available-for-sale investment securities [Abstract]
Derivative asset	0	0
Derivative liability	0	0
Recurring [Member] | Significant Unobservable Inputs, Level 3 [Member] | U.S. Treasury and agency securities
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0	0
Recurring [Member] | Significant Unobservable Inputs, Level 3 [Member] | Obligations of state and political subdivisions
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0	0
Recurring [Member] | Significant Unobservable Inputs, Level 3 [Member] | Government agency mortgage-backed securities
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0	0
Recurring [Member] | Significant Unobservable Inputs, Level 3 [Member] | Corporate debt securities
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0	0
Recurring [Member] | Significant Unobservable Inputs, Level 3 [Member] | Government agency collateralized mortgage obligations
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	$ 0	$ 0