FAIR VALUE MEASURES - Carrying Amount and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financial assets:
Cash and due from banks	$ 193,976	$ 172,036
Federal funds sold and interest-bearing deposits in banks	285,713	507,491
Time deposits in other banks	499	10,812
Loans, net	12,756,267	8,454,442
Accrued interest receivable	50,077	36,970
Financial liabilities:
Deposits	13,659,594	9,649,313
Securities sold under agreements to repurchase	17,744	20,384
Other borrowings	1,351,172	151,774
Subordinated deferrable interest debentures	127,075	89,187
FDIC loss-share payable	19,490	19,487
Accrued interest payable	11,107	5,669
Fair Value
Financial assets:
Cash and due from banks	193,976	172,036
Federal funds sold and interest-bearing deposits in banks	285,713	507,491
Time deposits in other banks	499	10,812
Loans, net	12,759,699	8,365,293
Accrued interest receivable	50,077	36,970
Financial liabilities:
Deposits	13,658,398	9,645,617
Securities sold under agreements to repurchase	17,744	20,384
Other borrowings	1,352,726	152,873
Subordinated deferrable interest debentures	124,130	90,180
FDIC loss-share payable	19,489	19,576
Accrued interest payable	11,107	5,669
Fair Value | Level 1
Financial assets:
Cash and due from banks	193,976	172,036
Federal funds sold and interest-bearing deposits in banks	285,713	507,491
Time deposits in other banks	0	0
Loans, net	0	0
Accrued interest receivable	0	0
Financial liabilities:
Deposits	0	0
Securities sold under agreements to repurchase	17,744	20,384
Other borrowings	0	0
Subordinated deferrable interest debentures	0	0
FDIC loss-share payable	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 deposits in banks	0	0
Time deposits in other banks	499	10,812
Loans, net	0	0
Accrued interest receivable	6,012	5,456
Financial liabilities:
Deposits	13,658,398	9,645,617
Securities sold under agreements to repurchase	0	0
Other borrowings	1,352,726	152,873
Subordinated deferrable interest debentures	124,130	90,180
FDIC loss-share payable	0	0
Accrued interest payable	11,107	5,669
Fair Value | Level 3
Financial assets:
Cash and due from banks	0	0
Federal funds sold and interest-bearing deposits in banks	0	0
Time deposits in other banks	0	0
Loans, net	12,759,699	8,365,293
Accrued interest receivable	44,065	31,514
Financial liabilities:
Deposits	0	0
Securities sold under agreements to repurchase	0	0
Other borrowings	0	0
Subordinated deferrable interest debentures	0	0
FDIC loss-share payable	19,489	19,576
Accrued interest payable	$ 0	$ 0