FAIR VALUE OF FINANCIAL INSTRUMENTS (Fair Value By Balance Sheet Location) (Details) - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Assets:
Securities—trading	$ 25,672,000	$ 25,896,000
Securities—available-for-sale	1,539,908,000	1,636,223,000
Securities—held-to-maturity	233,721,000	232,537,000
Liabilities:
FHLB advances at fair value	382,000,000	540,189,000
Carrying Value
Assets:
Cash and cash equivalents	324,456,000	272,196,000
Securities—trading	25,672,000	25,896,000
Securities—available-for-sale	1,539,908,000	1,636,223,000
Loans receivable held for sale	244,889,000	171,031,000
Loans receivable	8,835,368,000	8,684,595,000
FHLB stock	25,623,000	31,955,000
Bank-owned life insurance	179,076,000	177,467,000
Mortgage servicing rights	13,797,000	14,638,000
Equity Securities, FV-NI	0	352,000
Liabilities:
Demand, interest checking and money market accounts	6,646,341,000	6,314,202,000
Regular savings	1,863,839,000	1,842,581,000
Certificates of deposit	1,218,591,000	1,320,265,000
FHLB advances at fair value	382,000,000	540,189,000
Other borrowings	120,014,000	118,995,000
Junior subordinated debentures at fair value	113,417,000	114,091,000
Estimated Fair Value
Assets:
Cash and cash equivalents	324,456,000	272,196,000
Securities—trading	25,672,000	25,896,000
Securities—available-for-sale	1,539,908,000	1,636,223,000
Loans receivable held for sale	245,315,000	171,157,000
Loans receivable	8,890,803,000	8,629,450,000
FHLB stock	25,623,000	31,955,000
Bank-owned life insurance	179,076,000	177,467,000
Mortgage servicing rights	21,390,000	25,813,000
Equity Securities, FV-NI	0	352,000
Liabilities:
Demand, interest checking and money market accounts	6,646,341,000	6,314,202,000
Regular savings	1,863,839,000	1,842,581,000
Certificates of deposit	1,215,147,000	1,298,238,000
FHLB advances at fair value	385,521,000	540,189,000
Other borrowings	120,014,000	118,995,000
Junior subordinated debentures at fair value	113,417,000	114,091,000
Interest rate swaps [Member] | Carrying Value
Assets:
Derivatives:	19,289,000	3,138,000
Liabilities:
Derivatives:	14,399,000	3,138,000
Interest rate swaps [Member] | Estimated Fair Value
Assets:
Derivatives:	19,289,000	3,138,000
Liabilities:
Derivatives:	14,399,000	3,138,000
Interest Rate Forward Sales Commitments [Member] | Carrying Value
Assets:
Derivatives:	1,918,000	471,000
Liabilities:
Derivatives:	293,000	1,654,000
Interest Rate Forward Sales Commitments [Member] | Estimated Fair Value
Assets:
Derivatives:	1,918,000	471,000
Liabilities:
Derivatives:	293,000	1,654,000
Fair Value, Inputs, Level 2 [Member] | Carrying Value
Assets:
Securities—held-to-maturity	226,921,000	230,984,000
Fair Value, Inputs, Level 2 [Member] | Estimated Fair Value
Assets:
Securities—held-to-maturity	230,586,000	229,301,000
Fair Value, Inputs, Level 3 [Member] | Carrying Value
Assets:
Securities—held-to-maturity	3,135,000	3,236,000
Fair Value, Inputs, Level 3 [Member] | Estimated Fair Value
Assets:
Securities—held-to-maturity	$ 3,135,000	$ 3,236,000