FAIR VALUE MEASURES - Carrying Amount and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Financial assets:
Cash and due from banks	$ 224,159	$ 203,349
Federal funds sold and interest-bearing accounts	2,534,969	1,913,957
Time deposits in other banks	249	249
Loans, net	14,328,177	14,183,077
Accrued interest receivable	70,697	76,254
Financial liabilities:
Deposits	17,875,873	16,957,823
Securities sold under agreements to repurchase	9,320	11,641
Other borrowings	425,231	425,155
Subordinated deferrable interest debentures	124,833	124,345
Accrued interest payable	7,126	5,487
Fair Value
Financial assets:
Cash and due from banks	224,159	203,349
Federal funds sold and interest-bearing accounts	2,534,969	1,913,957
Time deposits in other banks	249	249
Loans, net	14,339,254	14,096,711
Accrued interest receivable	70,697	76,254
Financial liabilities:
Deposits	17,870,613	16,968,606
Securities sold under agreements to repurchase	9,320	11,641
Other borrowings	429,650	431,783
Subordinated deferrable interest debentures	117,285	116,280
Accrued interest payable	7,126	5,487
Fair Value | Level 1
Financial assets:
Cash and due from banks	224,159	203,349
Federal funds sold and interest-bearing accounts	2,534,969	1,913,957
Time deposits in other banks	0	0
Loans, net	0	0
Accrued interest receivable	0	0
Financial liabilities:
Deposits	0	0
Securities sold under agreements to repurchase	9,320	11,641
Other borrowings	0	0
Subordinated deferrable interest debentures	0	0
Accrued interest payable	0	0
Fair Value | Level 2
Financial assets:
Cash and due from banks	0	0
Federal funds sold and interest-bearing accounts	0	0
Time deposits in other banks	249	249
Loans, net	0	0
Accrued interest receivable	3,469	3,567
Financial liabilities:
Deposits	17,870,613	16,968,606
Securities sold under agreements to repurchase	0	0
Other borrowings	429,650	431,783
Subordinated deferrable interest debentures	117,285	116,280
Accrued interest payable	7,126	5,487
Fair Value | Level 3
Financial assets:
Cash and due from banks	0	0
Federal funds sold and interest-bearing accounts	0	0
Time deposits in other banks	0	0
Loans, net	14,339,254	14,096,711
Accrued interest receivable	67,228	72,687
Financial liabilities:
Deposits	0	0
Securities sold under agreements to repurchase	0	0
Other borrowings	0	0
Subordinated deferrable interest debentures	0	0
Accrued interest payable	$ 0	$ 0