FAIR VALUE - CARRYING AMOUNTS AND FV OF FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Assets:
Available-for-sale debt securities	$ 572,015		$ 475,738
Securities to be held to maturity	63,872		64,858
Equity securities with readily determinable fair value	3,367		2,806
Mortgage loans held for sale, at fair value	32,739	$ 13,883	8,971	$ 7,862	$ 12,653	$ 5,761
Consumer loans held for sale, at the lower of cost or fair value	18,504	$ 37,609	12,838	$ 21,037	$ 13,684	$ 8,551
Loans held for sale in connection with sale of banking centers, at the lower of cost or fair value	130,770
Loans, net	1,187		1,922
Liabilities:
Deposits held for assumption in connection with sale of banking centers	142,384
Carrying Value
Assets:
Cash and cash equivalents	397,072		351,474
Available-for-sale debt securities	572,015		475,738
Securities to be held to maturity	63,315		65,227
Equity securities with readily determinable fair value	3,367		2,806
Mortgage loans held for sale, at fair value	32,739		8,971
Consumer loans held for sale, at the lower of cost or fair value	18,504		12,838
Loans held for sale in connection with sale of banking centers, at the lower of cost or fair value	130,770
Loans, net	4,617,122		4,103,552
Federal Home Loan Bank stock	32,242		32,067
Accrued interest receivable	13,699		13,942
Rate lock loan commitments	1,193		356
Mandatory forward contracts	211
Interest rate swap agreements	6,483		1,264
Liabilities:
Deposits held for assumption in connection with sale of banking centers	142,384
Securities sold under agreements to repurchase and other short-term borrowings	167,949		182,990
Federal Home Loan Bank advances	1,170,000		810,000
Subordinated note	41,240		41,240
Accrued interest payable	2,362		1,084
Mandatory forward contracts			262
Interest rate swap agreements	6,620		1,149
Carrying Value | Non Interest Bearing Deposits
Liabilities:
Deposit liabilities, fair value	1,031,553		1,003,969
Carrying Value | Transaction deposits
Liabilities:
Deposit liabilities, fair value	2,109,821		2,035,701
Carrying Value | Time deposits.
Liabilities:
Deposit liabilities, fair value	593,378		416,475
Total Fair Value
Assets:
Cash and cash equivalents	397,072		351,474
Available-for-sale debt securities	572,015		475,738
Securities to be held to maturity	63,872		64,858
Equity securities with readily determinable fair value	3,367		2,806
Mortgage loans held for sale, at fair value	32,739		8,971
Consumer loans held for sale, at the lower of cost or fair value	18,504		12,838
Loans held for sale in connection with sale of banking centers, at the lower of cost or fair value	130,770
Loans, net	4,695,637		4,062,457
Accrued interest receivable	13,699		13,942
Rate lock loan commitments	1,193		356
Mandatory forward contracts	211
Interest rate swap agreements	6,483		1,264
Liabilities:
Deposits held for assumption in connection with sale of banking centers	142,563
Securities sold under agreements to repurchase and other short-term borrowings	167,949		182,990
Federal Home Loan Bank advances	1,169,392		804,251
Subordinated note	31,421		33,724
Accrued interest payable	2,362		1,084
Mandatory forward contracts			262
Interest rate swap agreements	6,620		1,149
Total Fair Value | Non Interest Bearing Deposits
Liabilities:
Deposit liabilities, fair value	1,031,553		1,003,969
Total Fair Value | Transaction deposits
Liabilities:
Deposit liabilities, fair value	2,109,821		2,035,701
Total Fair Value | Time deposits.
Liabilities:
Deposit liabilities, fair value	597,044		412,477
Total Fair Value | Fair Value, Inputs, Level 1
Assets:
Cash and cash equivalents	397,072		351,474
Equity securities with readily determinable fair value	2,485		2,396
Total Fair Value | Fair Value, Inputs, Level 2
Assets:
Available-for-sale debt securities	564,420		467,951
Securities to be held to maturity	63,872		64,858
Equity securities with readily determinable fair value	882		410
Mortgage loans held for sale, at fair value	32,739		8,971
Consumer loans held for sale, at the lower of cost or fair value	18,504		12,838
Loans held for sale in connection with sale of banking centers, at the lower of cost or fair value	130,770
Accrued interest receivable	13,699		13,942
Rate lock loan commitments	1,193		356
Mandatory forward contracts	211
Interest rate swap agreements	6,483		1,264
Liabilities:
Deposits held for assumption in connection with sale of banking centers	142,563
Securities sold under agreements to repurchase and other short-term borrowings	167,949		182,990
Federal Home Loan Bank advances	1,169,392		804,251
Subordinated note	31,421		33,724
Accrued interest payable	2,362		1,084
Mandatory forward contracts			262
Interest rate swap agreements	6,620		1,149
Total Fair Value | Fair Value, Inputs, Level 2 | Non Interest Bearing Deposits
Liabilities:
Deposit liabilities, fair value	1,031,553		1,003,969
Total Fair Value | Fair Value, Inputs, Level 2 | Transaction deposits
Liabilities:
Deposit liabilities, fair value	2,109,821		2,035,701
Total Fair Value | Fair Value, Inputs, Level 2 | Time deposits.
Liabilities:
Deposit liabilities, fair value	597,044		412,477
Total Fair Value | Fair Value, Inputs, Level 3
Assets:
Available-for-sale debt securities	7,595		7,787
Loans, net	$ 4,695,637		$ 4,062,457