FAIR VALUE MEASURES - Carrying Amount and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Financial assets:
Cash and due from banks	$ 292,899	$ 246,234
Federal funds sold and interest-bearing accounts	428,560	375,615
Time deposits in other banks	249	249
Loans, net	14,201,538	12,736,499
Accrued interest receivable	72,175	52,362
Financial liabilities:
Deposits	15,589,818	14,027,073
Securities sold under agreements to repurchase	12,879	20,635
Other borrowings	1,418,336	1,398,709
Subordinated deferrable interest debentures	123,375	127,560
FDIC loss-share payable	18,903	19,642
Accrued interest payable	6,862	11,524
Fair Value
Financial assets:
Cash and due from banks	292,899	246,234
Federal funds sold and interest-bearing accounts	428,560	375,615
Time deposits in other banks	249	249
Loans, net	14,249,663	12,806,709
Accrued interest receivable	72,175	52,362
Financial liabilities:
Deposits	15,611,958	14,035,686
Securities sold under agreements to repurchase	12,879	20,635
Other borrowings	1,424,093	1,402,510
Subordinated deferrable interest debentures	116,869	126,815
FDIC loss-share payable	19,085	19,657
Accrued interest payable	6,826	11,524
Fair Value | Level 1
Financial assets:
Cash and due from banks	292,899	246,234
Federal funds sold and interest-bearing accounts	428,560	375,615
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	12,879	20,635
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 accounts	0	0
Time deposits in other banks	249	249
Loans, net	0	0
Accrued interest receivable	4,490	5,179
Financial liabilities:
Deposits	15,611,958	14,035,686
Securities sold under agreements to repurchase	0	0
Other borrowings	1,424,093	1,402,510
Subordinated deferrable interest debentures	116,869	126,815
FDIC loss-share payable	0	0
Accrued interest payable	6,826	11,524
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	0
Loans, net	14,249,663	12,806,709
Accrued interest receivable	67,685	47,183
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	19,085	19,657
Accrued interest payable	$ 0	$ 0