FAIR VALUE OF FINANCIAL INSTRUMENTS (Assets and Liabilities Measured at Fair Value) (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Securities, Fair Value	$ 25,636,000	$ 25,896,000
Securities—available-for-sale	1,551,557,000	1,636,223,000
Debt Securities, Held-to-maturity, Fair Value	237,805,000	232,537,000
Loans receivable held for sale	199,400,000	164,800,000
Advances from FHLB	450,000,000	540,189,000
Reported Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	307,735,000	272,196,000
Trading Securities, Fair Value	25,636,000	25,896,000
Securities—available-for-sale	1,551,557,000	1,636,223,000
Loans receivable held for sale	210,447,000	171,031,000
Loans receivable	9,305,357,000	8,684,595,000
FHLB stock	28,342,000	31,955,000
Bank-owned life insurance	192,088,000	177,467,000
MSRs	14,148,000	14,638,000
Equity Securities, FV-NI	0	352,000
Deposits, Demand NOW and Money Market Accounts, Fair Value Disclosure	6,994,197,000	6,314,202,000
Deposits, Savings, Fair Value Disclosure	1,934,041,000	1,842,581,000
Deposits, Certificates of Deposit, Fair Value Disclosure	1,120,403,000	1,320,265,000
Advances from FHLB	450,000,000	540,189,000
Junior subordinated debentures net of unamortized deferred issuance costs at fair value	119,304,000	114,091,000
Other borrowings	118,474,000	118,995,000
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	1,108,000	471,000
Derivative liabilities	674,000	1,654,000
Reported Value Measurement [Member] | Interest Rate Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	15,202,000	3,138,000
Derivative liabilities	10,966,000	3,138,000
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	233,241,000	230,984,000
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	2,853,000	3,236,000
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	307,735,000	272,196,000
Trading Securities, Fair Value	25,636,000	25,896,000
Securities—available-for-sale	1,551,557,000	1,636,223,000
Loans receivable held for sale	210,670,000	171,157,000
Loans receivable	9,304,340,000	8,629,450,000
FHLB stock	28,342,000	31,955,000
Bank-owned life insurance	192,088,000	177,467,000
MSRs	22,611,000	25,813,000
Equity Securities, FV-NI	0	352,000
Deposits, Demand NOW and Money Market Accounts, Fair Value Disclosure	6,994,197,000	6,314,202,000
Deposits, Savings, Fair Value Disclosure	1,934,041,000	1,842,581,000
Deposits, Certificates of Deposit, Fair Value Disclosure	1,117,921,000	1,298,238,000
Advances from FHLB	452,720,000	540,189,000
Junior subordinated debentures net of unamortized deferred issuance costs at fair value	119,304,000	114,091,000
Other borrowings	118,474,000	118,995,000
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	1,108,000	471,000
Derivative liabilities	674,000	1,654,000
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	15,202,000	3,138,000
Derivative liabilities	10,966,000	3,138,000
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	234,952,000	229,301,000
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	$ 2,853,000	$ 3,236,000