FAIR VALUE OF FINANCIAL INSTRUMENTS (Fair Value By Balance Sheet Location) (Details) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Assets:
Securities—trading	$ 25,640,000	$ 22,318,000
Securities—available-for-sale	1,400,312,000	919,485,000
Securities—held-to-maturity	260,318,000	262,188,000
Liabilities:
FHLB advances at fair value	239,190,000	202,000
Carrying Value
Assets:
Cash and cash equivalents	249,425,000	261,200,000
Securities—trading	25,640,000	22,318,000
Securities—available-for-sale	1,400,312,000	919,485,000
Loans receivable held for sale	78,833,000	40,725,000
Loans receivable	7,684,732,000	7,598,884,000
FHLB stock	19,916,000	10,334,000
Bank-owned life insurance	164,225,000	162,668,000
Mortgage servicing rights	14,521,000	14,738,000
Marketable Securities, Equity Securities	461,000	0
Liabilities:
Demand, interest checking and money market accounts	5,743,980,000	5,658,994,000
Regular savings	1,635,080,000	1,557,500,000
Certificates of deposit	1,148,607,000	966,937,000
FHLB advances at fair value	239,190,000	202,000
Other borrowings	112,458,000	95,860,000
Junior subordinated debentures at fair value	112,774,000	98,707,000
Estimated Fair Value
Assets:
Cash and cash equivalents	249,425,000	261,200,000
Securities—trading	25,640,000	22,318,000
Securities—available-for-sale	1,400,312,000	919,485,000
Loans receivable held for sale	78,950,000	40,923,000
Loans receivable	7,567,722,000	7,445,990,000
FHLB stock	19,916,000	10,334,000
Bank-owned life insurance	164,225,000	162,668,000
Mortgage servicing rights	23,363,000	19,835,000
Marketable Securities, Equity Securities	461,000	0
Liabilities:
Demand, interest checking and money market accounts	5,743,980,000	5,658,994,000
Regular savings	1,635,080,000	1,557,500,000
Certificates of deposit	1,133,081,000	947,517,000
FHLB advances at fair value	239,190,000	202,000
Other borrowings	112,458,000	95,860,000
Junior subordinated debentures at fair value	112,774,000	98,707,000
Interest rate swaps [Member] | Carrying Value
Assets:
Derivatives:	5,682,000	5,083,000
Liabilities:
Derivatives:	5,682,000	5,083,000
Interest rate swaps [Member] | Estimated Fair Value
Assets:
Derivatives:	5,682,000	5,083,000
Liabilities:
Derivatives:	5,682,000	5,083,000
Interest Rate Forward Sales Commitments [Member] | Carrying Value
Assets:
Derivatives:	501,000	523,000
Liabilities:
Derivatives:	416,000	201,000
Interest Rate Forward Sales Commitments [Member] | Estimated Fair Value
Assets:
Derivatives:	501,000	523,000
Liabilities:
Derivatives:	416,000	201,000
Fair Value, Inputs, Level 2 [Member] | Carrying Value
Assets:
Securities—held-to-maturity	259,871,000	256,793,000
Fair Value, Inputs, Level 2 [Member] | Estimated Fair Value
Assets:
Securities—held-to-maturity	257,013,000	258,710,000
Fair Value, Inputs, Level 3 [Member] | Carrying Value
Assets:
Securities—held-to-maturity	3,305,000	3,478,000
Fair Value, Inputs, Level 3 [Member] | Estimated Fair Value
Assets:
Securities—held-to-maturity	$ 3,305,000	$ 3,478,000