FAIR VALUE MEASURES - Carrying Amount and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Financial assets:
Cash and due from banks	$ 259,729	$ 203,349
Federal funds sold and interest-bearing accounts	3,044,795	1,913,957
Time deposits in other banks	0	249
Investment securities held-to-maturity	29,008	0
Financial liabilities:
Other borrowings	425,303	425,155
Subordinated deferrable interest debentures	125,331	124,345
Carrying Amount
Financial assets:
Cash and due from banks	259,729	203,349
Federal funds sold and interest-bearing accounts	3,044,795	1,913,957
Time deposits in other banks	0	249
Investment securities held-to-maturity	29,055	0
Loans, net	14,527,191	14,183,077
Accrued interest receivable	64,906	76,254
Financial liabilities:
Deposits	18,257,997	16,957,823
Securities sold under agreements to repurchase	5,544	11,641
Other borrowings	425,303	425,155
Subordinated deferrable interest debentures	125,331	124,345
Accrued interest payable	4,375	5,487
Fair Value
Financial assets:
Cash and due from banks	259,729	203,349
Federal funds sold and interest-bearing accounts	3,044,795	1,913,957
Time deposits in other banks	0	249
Investment securities held-to-maturity	29,008	0
Loans, net	14,407,763	14,096,711
Accrued interest receivable	64,906	76,254
Financial liabilities:
Deposits	18,262,653	16,968,606
Securities sold under agreements to repurchase	5,544	11,641
Other borrowings	430,674	431,783
Subordinated deferrable interest debentures	117,735	116,280
Accrued interest payable	4,375	5,487
Fair Value | Level 1
Financial assets:
Cash and due from banks	259,729	203,349
Federal funds sold and interest-bearing accounts	3,044,795	1,913,957
Time deposits in other banks	0	0
Investment securities held-to-maturity	0	0
Loans, net	0	0
Accrued interest receivable	0	0
Financial liabilities:
Deposits	0	0
Securities sold under agreements to repurchase	5,544	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	0	249
Investment securities held-to-maturity	29,008	0
Loans, net	0	0
Accrued interest receivable	2,919	3,567
Financial liabilities:
Deposits	18,262,653	16,968,606
Securities sold under agreements to repurchase	0	0
Other borrowings	430,674	431,783
Subordinated deferrable interest debentures	117,735	116,280
Accrued interest payable	4,375	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
Investment securities held-to-maturity	0	0
Loans, net	14,407,763	14,096,711
Accrued interest receivable	61,987	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