FAIR VALUE MEASURES - Carrying Amount and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Financial assets:
Cash and due from banks	$ 239,028	$ 203,349
Federal funds sold and interest-bearing accounts	3,513,412	1,913,957
Time deposits in other banks	0	249
Investment securities held-to-maturity	63,304	0
Financial liabilities:
Other borrowings	425,375	425,155
Subordinated deferrable interest debentures	125,830	124,345
Carrying Amount
Financial assets:
Cash and due from banks	239,028	203,349
Federal funds sold and interest-bearing accounts	3,513,412	1,913,957
Time deposits in other banks		249
Investment securities held-to-maturity	64,451	0
Loans, net	14,594,746	14,183,077
Accrued interest receivable	59,957	76,254
Financial liabilities:
Deposits	18,833,489	16,957,823
Securities sold under agreements to repurchase	4,502	11,641
Other borrowings	425,375	425,155
Subordinated deferrable interest debentures	125,830	124,345
Accrued interest payable	6,356	5,487
Fair Value
Financial assets:
Cash and due from banks	239,028	203,349
Federal funds sold and interest-bearing accounts	3,513,412	1,913,957
Time deposits in other banks		249
Investment securities held-to-maturity	63,304	0
Loans, net	14,574,784	14,096,711
Accrued interest receivable	59,957	76,254
Financial liabilities:
Deposits	18,836,507	16,968,606
Securities sold under agreements to repurchase	4,502	11,641
Other borrowings	430,037	431,783
Subordinated deferrable interest debentures	117,283	116,280
Accrued interest payable	6,356	5,487
Fair Value | Level 1
Financial assets:
Cash and due from banks	239,028	203,349
Federal funds sold and interest-bearing accounts	3,513,412	1,913,957
Time deposits in other banks		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	4,502	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
Investment securities held-to-maturity	63,304	0
Loans, net	0	0
Accrued interest receivable	2,991	3,567
Financial liabilities:
Deposits	18,836,507	16,968,606
Securities sold under agreements to repurchase	0	0
Other borrowings	430,037	431,783
Subordinated deferrable interest debentures	117,283	116,280
Accrued interest payable	6,356	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
Investment securities held-to-maturity	0	0
Loans, net	14,574,784	14,096,711
Accrued interest receivable	56,966	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