FAIR VALUE OF FINANCIAL INSTRUMENTS (Assets and Liabilities Measured at Fair Value) (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Securities, Fair Value	$ 25,896,000	$ 22,318,000
Securities—available-for-sale	1,636,223,000	919,485,000
Debt Securities, Held-to-maturity, Fair Value	232,537,000	262,188,000
Loans receivable held for sale	164,800,000	32,400,000
Advances from FHLB	540,189,000	202,000
Reported Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	272,196,000	261,200,000
Trading Securities, Fair Value	25,896,000	22,318,000
Securities—available-for-sale	1,636,223,000	919,485,000
Loans receivable held for sale	171,031,000	40,725,000
Loans receivable	8,684,595,000	7,598,884,000
FHLB stock	31,955,000	10,334,000
Bank-owned life insurance	177,467,000	162,668,000
MSRs	14,638,000	14,738,000
Equity Securities, FV-NI	352,000	0
Deposits, Demand NOW and Money Market Accounts, Fair Value Disclosure	6,314,202,000	5,658,994,000
Deposits, Savings, Fair Value Disclosure	1,842,581,000	1,557,500,000
Deposits, Certificates of Deposit, Fair Value Disclosure	1,320,265,000	966,937,000
Advances from FHLB	540,189,000	202,000
Junior subordinated debentures net of unamortized deferred issuance costs at fair value	114,091,000	98,707,000
Other borrowings	118,995,000	95,860,000
Reported Value Measurement [Member] | Interest Rate Forward Sales Commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	471,000	523,000
Derivative liabilities	1,654,000	201,000
Reported Value Measurement [Member] | Interest Rate Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	3,138,000	5,083,000
Derivative liabilities	3,138,000	5,083,000
Reported Value Measurement [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Held-to-maturity, Fair Value	230,984,000	256,793,000
Reported Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Held-to-maturity, Fair Value	3,236,000	3,478,000
Estimate of Fair Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	272,196,000	261,200,000
Trading Securities, Fair Value	25,896,000	22,318,000
Securities—available-for-sale	1,636,223,000	919,485,000
Loans receivable held for sale	171,157,000	40,923,000
Loans receivable	8,629,450,000	7,445,990,000
FHLB stock	31,955,000	10,334,000
Bank-owned life insurance	177,467,000	162,668,000
MSRs	25,813,000	19,835,000
Equity Securities, FV-NI	352,000	0
Deposits, Demand NOW and Money Market Accounts, Fair Value Disclosure	6,314,202,000	5,658,994,000
Deposits, Savings, Fair Value Disclosure	1,842,581,000	1,557,500,000
Deposits, Certificates of Deposit, Fair Value Disclosure	1,298,238,000	947,517,000
Advances from FHLB	540,189,000	202,000
Junior subordinated debentures net of unamortized deferred issuance costs at fair value	114,091,000	98,707,000
Other borrowings	118,995,000	95,860,000
Estimate of Fair Value Measurement [Member] | Interest Rate Forward Sales Commitments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	471,000	523,000
Derivative liabilities	1,654,000	201,000
Estimate of Fair Value Measurement [Member] | Interest Rate Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	3,138,000	5,083,000
Derivative liabilities	3,138,000	5,083,000
Estimate of Fair Value Measurement [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Held-to-maturity, Fair Value	229,301,000	258,710,000
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Held-to-maturity, Fair Value	$ 3,236,000	$ 3,478,000