FAIR VALUE MEASURES - Carrying Amount and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Financial assets:
Cash and due from banks	$ 257,316	$ 307,813
Federal funds sold and interest-bearing accounts	3,541,144	3,756,844
Debt securities held-to-maturity	82,489	78,206
Financial liabilities:
Other borrowings	425,520	739,879
Subordinated deferrable interest debentures	126,827	126,328
Carrying Amount
Financial assets:
Cash and due from banks	257,316	307,813
Federal funds sold and interest-bearing accounts	3,541,144	3,756,844
Debt securities held-to-maturity	91,454	79,850
Loans, net	15,957,040	15,662,495
Accrued interest receivable	53,036	56,917
Financial liabilities:
Deposits	19,588,441	19,665,553
Securities sold under agreements to repurchase	2,065	5,845
Other borrowings	425,520	739,879
Subordinated deferrable interest debentures	126,827	126,328
Accrued interest payable	6,121	4,313
Fair Value
Financial assets:
Cash and due from banks	257,316	307,813
Federal funds sold and interest-bearing accounts	3,541,144	3,756,844
Debt securities held-to-maturity	82,489	78,206
Loans, net	15,927,461	15,509,410
Accrued interest receivable	53,036	56,917
Financial liabilities:
Deposits	19,590,158	19,667,612
Securities sold under agreements to repurchase	2,065	5,845
Other borrowings	431,131	760,829
Subordinated deferrable interest debentures	117,196	117,764
Accrued interest payable	6,121	4,313
Fair Value | Level 1
Financial assets:
Cash and due from banks	257,316	307,813
Federal funds sold and interest-bearing accounts	3,541,144	3,756,844
Debt 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	2,065	5,845
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
Debt securities held-to-maturity	82,489	78,206
Loans, net	0	0
Accrued interest receivable	2,598	2,373
Financial liabilities:
Deposits	19,590,158	19,667,612
Securities sold under agreements to repurchase	0	0
Other borrowings	431,131	760,829
Subordinated deferrable interest debentures	117,196	117,764
Accrued interest payable	6,121	4,313
Fair Value | Level 3
Financial assets:
Cash and due from banks	0	0
Federal funds sold and interest-bearing accounts	0	0
Debt securities held-to-maturity	0	0
Loans, net	15,927,461	15,509,410
Accrued interest receivable	50,438	54,544
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