OTHER BORROWINGS - Schedule of Other Borrowings (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Other Borrowings [Line Items]
Subordinated deferrable interest debentures	$ 123,860	$ 127,560
Other borrowings	875,255	1,398,709
Fixed Rate Advance due January 10, 2020; fixed interest rate of 1.68%
Other Borrowings [Line Items]
Advance from correspondent bank	0	$ 50,000
Debt instrument, interest rate, effective percentage		1.68%
Fixed Rate Advance due January 13, 2020; fixed interest rate of 1.68%
Other Borrowings [Line Items]
Advance from correspondent bank	0	$ 50,000
Debt instrument, interest rate, effective percentage		1.68%
Fixed Rate Advance due January 13, 2020; fixed interest rate of 1.67%
Other Borrowings [Line Items]
Advance from correspondent bank	0	$ 100,000
Debt instrument, interest rate, effective percentage		1.67%
Fixed Rate Advance due January 15, 2020; fixed interest rate of 1.71%
Other Borrowings [Line Items]
Advance from correspondent bank	0	$ 50,000
Debt instrument, interest rate, effective percentage		1.71%
Fixed Rate Advance due January 16, 2020; fixed interest rate of 1.69%
Other Borrowings [Line Items]
Advance from correspondent bank	0	$ 150,000
Debt instrument, interest rate, effective percentage		1.69%
Fixed Rate Advance due January 17, 2020; fixed interest rate of 1.70%
Other Borrowings [Line Items]
Advance from correspondent bank	0	$ 100,000
Debt instrument, interest rate, effective percentage		1.70%
Fixed Rate Advance due January 21, 2020; fixed interest rate of 1.71%
Other Borrowings [Line Items]
Advance from correspondent bank	0	$ 50,000
Debt instrument, interest rate, effective percentage		1.71%
Fixed Rate Advance due January 21, 2020; fixed interest rate of 1.71%
Other Borrowings [Line Items]
Advance from correspondent bank	0	$ 200,000
Debt instrument, interest rate, effective percentage		1.71%
Fixed Rate Advance due January 21, 2020; fixed interest rate of 1.70%
Other Borrowings [Line Items]
Advance from correspondent bank	0	$ 25,000
Debt instrument, interest rate, effective percentage		1.70%
Fixed Rate Advance due January 21, 2020; fixed interest rate of 1.71%
Other Borrowings [Line Items]
Advance from correspondent bank	0	$ 75,000
Debt instrument, interest rate, effective percentage		1.71%
Fixed Rate Advance due January 21, 2020; fixed interest rate of 1.71%
Other Borrowings [Line Items]
Advance from correspondent bank	0	$ 25,000
Debt instrument, interest rate, effective percentage		1.71%
Fixed Rate Advance due January 23, 2020; fixed interest rate of 1.71%
Other Borrowings [Line Items]
Advance from correspondent bank	0	$ 100,000
Debt instrument, interest rate, effective percentage		1.71%
Fixed Rate Advance due January 27, 2020; fixed interest rate of 1.73%
Other Borrowings [Line Items]
Advance from correspondent bank	0	$ 50,000
Debt instrument, interest rate, effective percentage		1.73%
Fixed Rate Advance due February 18, 2020; fixed interest rate of 1.72%
Other Borrowings [Line Items]
Advance from correspondent bank	0	$ 100,000
Debt instrument, interest rate, effective percentage		1.72%
Fixed Rate Advance due October 5, 2020; fixed interest rate of 0.22%
Other Borrowings [Line Items]
Advance from correspondent bank	$ 200,000	$ 0
Debt instrument, interest rate, effective percentage	0.22%
Fixed Rate Advance due October 13, 2020; fixed interest rate of 0.22%
Other Borrowings [Line Items]
Advance from correspondent bank	$ 150,000	0
Debt instrument, interest rate, effective percentage	0.22%
Fixed Rate Advance due October 26, 2020; fixed interest rate of 0.22%
Other Borrowings [Line Items]
Advance from correspondent bank	$ 100,000	0
Debt instrument, interest rate, effective percentage	0.22%
Fixed Rate Advance due March 3, 2025; fixed interest rate of 1.208%
Other Borrowings [Line Items]
Advance from correspondent bank	$ 15,000	0
Debt instrument, interest rate, effective percentage	1.208%
Fixed Rate Advance due March 2, 2027; fixed interest rate of 1.445%
Other Borrowings [Line Items]
Advance from correspondent bank	$ 15,000	0
Debt instrument, interest rate, effective percentage	1.445%
Fixed Rate Advance due March 4, 2030; fixed interest rate of 1.606%
Other Borrowings [Line Items]
Advance from correspondent bank	$ 15,000	0
Debt instrument, interest rate, effective percentage	1.606%
Fixed Rate Advance due December 9, 2030; fixed interest rate of 4.55%
Other Borrowings [Line Items]
Advance from correspondent bank	$ 1,414	1,422
Debt instrument, interest rate, effective percentage	4.55%
Fixed Rate Advance due December 9, 2030; fixed interest rate of 4.55%
Other Borrowings [Line Items]
Advance from correspondent bank	$ 979	985
Debt instrument, interest rate, effective percentage	4.55%
Principal Reducing Advance due September 29, 2031; fixed interest rate of 3.095%
Other Borrowings [Line Items]
Advance from correspondent bank	$ 1,603	1,712
Debt instrument, interest rate, effective percentage	3.095%
Subordinated notes payable due March 15, 2027 net of unamortized debt issuance cost of $845 and $943, respectively; fixed interest rate of 5.75% through March 14, 2022; variable interest rate thereafter at three-month LIBOR plus 3.616%
Other Borrowings [Line Items]
Subordinated deferrable interest debentures	$ 74,155	74,057
Debt instrument, interest rate, effective percentage	5.75%
Unamortized debt issuance expense	$ 845	943
Subordinated notes payable due March 15, 2027 net of unamortized debt issuance cost of $845 and $943, respectively; fixed interest rate of 5.75% through March 14, 2022; variable interest rate thereafter at three-month LIBOR plus 3.616% | London Interbank Offered Rate (LIBOR)
Other Borrowings [Line Items]
Basis spread on variable rate	3.616%
Subordinated notes payable due December 15, 2029 net of unamortized debt issuance cost of $2,226 and $2,408, respectively; fixed interest rate of 4.25% through December 14, 2024; variable interest rate thereafter at three-month SOFR plus 2.94%
Other Borrowings [Line Items]
Subordinated deferrable interest debentures	$ 117,774	117,592
Debt instrument, interest rate, effective percentage	4.25%
Unamortized debt issuance expense	$ 2,226	2,408
Subordinated notes payable due December 15, 2029 net of unamortized debt issuance cost of $2,226 and $2,408, respectively; fixed interest rate of 4.25% through December 14, 2024; variable interest rate thereafter at three-month SOFR plus 2.94% | Secured Overnight Financing Rate (SOFR)
Other Borrowings [Line Items]
Basis spread on variable rate	2.94%
Subordinated notes payable due May 31, 2030 net of unaccreted purchase accounting fair value adjustment of $1,181 and $1,596, respectively; fixed interest rate of 5.875% through May 31, 2025; variable interest rate thereafter at three-month LIBOR plus 3.63%
Other Borrowings [Line Items]
Subordinated deferrable interest debentures	$ 76,181	76,595
Debt instrument, interest rate, effective percentage	5.875%
Unaccreted purchase accounting fair value adjustment	$ 1,181	1,596
Subordinated notes payable due May 31, 2030 net of unaccreted purchase accounting fair value adjustment of $1,181 and $1,596, respectively; fixed interest rate of 5.875% through May 31, 2025; variable interest rate thereafter at three-month LIBOR plus 3.63% | London Interbank Offered Rate (LIBOR)
Other Borrowings [Line Items]
Basis spread on variable rate	3.63%
Subordinated notes payable due October 1, 2030 net of unamortized debt issuance cost of $1,851 and $0, respectively; fixed interest rate of 3.875% through September 30, 2025; variable interest rate thereafter at three-month SOFR plus 3.753%
Other Borrowings [Line Items]
Subordinated deferrable interest debentures	$ 108,149	0
Debt instrument, interest rate, effective percentage	3.875%
Unamortized debt issuance expense	$ 1,851	0
Subordinated notes payable due October 1, 2030 net of unamortized debt issuance cost of $1,851 and $0, respectively; fixed interest rate of 3.875% through September 30, 2025; variable interest rate thereafter at three-month SOFR plus 3.753% | London Interbank Offered Rate (LIBOR)
Other Borrowings [Line Items]
Basis spread on variable rate	3.753%
Advance from correspondent bank due September 5, 2026; secured by a loan receivable; fixed interest rate of 2.09%
Other Borrowings [Line Items]
Other borrowings	$ 0	$ 1,346
Debt instrument, interest rate, effective percentage	2.09%