OTHER BORROWINGS - Schedule of Other Borrowings (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Other Borrowings [Line Items]
Net carrying value of subordinated debt	$ 88,646	$ 85,550
Other borrowings	862,136	250,554
Daily Rate Credit with a variable interest rate (2.16% at June 30, 2018 and 1.59% at December 31, 2017)
Other Borrowings [Line Items]
Advance from correspondent bank	$ 145,000	$ 25,000
Debt instrument, interest rate, effective percentage	2.16%	1.59%
Fixed Rate Advance due July 11, 2018; fixed interest rate of 1.98%
Other Borrowings [Line Items]
Advance from correspondent bank	$ 425,000	$ 0
Debt instrument, interest rate, effective percentage	1.98%
Fixed Rate Advance due July 19, 2018; fixed interest rate of 2.01%
Other Borrowings [Line Items]
Advance from correspondent bank	$ 135,000	0
Debt instrument, interest rate, effective percentage	2.01%
Fixed Rate Hybrid Advance due November 6, 2018; fixed interest rate of 2.727%
Other Borrowings [Line Items]
Advance from correspondent bank	$ 5,000	0
Debt instrument, interest rate, effective percentage	2.73%
Convertible Flipper Advance due May 22, 2019; current interest rate of 4.68%
Other Borrowings [Line Items]
Advance from correspondent bank	$ 1,500	0
Debt instrument, interest rate, effective percentage	4.68%
Principal Reducing Advance due June 20, 2019; fixed interest rate of 1.274%
Other Borrowings [Line Items]
Advance from correspondent bank	$ 1,000	0
Fixed Rate Advance due December 9, 2030; fixed interest rate of 4.55%
Other Borrowings [Line Items]
Advance from correspondent bank	1,300	0
Fixed Rate Advance due December 9, 2030; fixed interest rate of 4.55%
Other Borrowings [Line Items]
Advance from correspondent bank	900	0
Principal Reducing Advance due September 29, 2031; fixed interest rate of 3.095%
Other Borrowings [Line Items]
Advance from correspondent bank	1,921	0
Fixed Rate Advance due January 8, 2018; fixed interest rate of 1.39%
Other Borrowings [Line Items]
Advance from correspondent bank	0	150,000
Subordinated notes payable due March 15, 2027 net of unamortized debt issuance cost of $1,140 and $1,205, 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]
Net carrying value of subordinated debt	$ 73,860	73,795
Debt instrument, interest rate, effective percentage	5.75%
Unamortized debt issuance expense	$ 1,205	1,140
Advance from correspondent bank due October 5, 2019; secured by a loan receivable; fixed interest rate of 4.25%
Other Borrowings [Line Items]
Other borrowings	$ 35	49
Debt instrument, interest rate, effective percentage	4.25%
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	$ 1,620	1,710
Debt instrument, interest rate, effective percentage	2.09%
Advances under revolving credit agreement with a regional bank due September 26, 2020; secured by subsidiary bank stock; variable interest rate at 90-day LIBOR plus 3.50% (5.80% at June 30, 2018)
Other Borrowings [Line Items]
Long-term line of credit	$ 70,000	$ 0
Debt instrument, interest rate, effective percentage	5.80%
Basis spread on variable rate	3.50%
London Interbank Offered Rate (LIBOR) | Subordinated notes payable due March 15, 2027 net of unamortized debt issuance cost of $1,140 and $1,205, 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]
Basis spread on variable rate	3.616%