LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Risk-Rated and Non-Risk Rated Loans by Grade and Other Characteristic) (Details) - USD ($) $ in Thousands		Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		$ 7,398,637	$ 7,314,504	$ 4,372,594
Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		3,259,216	3,093,160	1,697,564
Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		266,883	472,976	198,874
Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		797,327	574,370	493,800
Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		1,187,848	1,207,944	812,070
Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		383,275	376,531	242,556
One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		846,899	952,633	536,325
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		657,189	636,890	$ 391,405
Pass (Risk Ratings 1-5) [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[1]	7,240,970	7,148,698
Pass (Risk Ratings 1-5) [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[1]	3,184,915	3,022,281
Pass (Risk Ratings 1-5) [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[1]	264,899	468,467
Pass (Risk Ratings 1-5) [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[1]	783,363	558,425
Pass (Risk Ratings 1-5) [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[1]	1,140,359	1,167,933
Pass (Risk Ratings 1-5) [Member] | Small Credit-Scored Business Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		202,200	150,000
Pass (Risk Ratings 1-5) [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[1]	372,727	354,760
Pass (Risk Ratings 1-5) [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[1]	840,609	943,098
Pass (Risk Ratings 1-5) [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[1]	654,098	633,734
Special mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		63,028	77,632
Special mention [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		35,144	30,928
Special mention [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		582	138
Special mention [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		2,966	2,386
Special mention [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		20,529	25,286
Special mention [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		2,648	17,526
Special mention [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		1,026	1,346
Special mention [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		133	22
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		94,632	88,164
Substandard [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		39,157	39,951
Substandard [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		1,402	4,371
Substandard [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		10,998	13,559
Substandard [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		26,960	14,725
Substandard [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		7,900	4,245
Substandard [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		5,264	8,189
Substandard [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		2,951	3,124
Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		7	10
Doubtful [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Doubtful [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Doubtful [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Doubtful [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Doubtful [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Doubtful [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Doubtful [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		7	10
Loss [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Loss [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Loss [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Loss [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Loss [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Loss [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Loss [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Loss [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		0	0
Performing loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		7,332,667	7,240,682
Performing loans [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		3,217,896	3,048,424
Performing loans [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		266,448	470,982
Performing loans [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		791,427	566,460
Performing loans [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		1,180,819	1,198,475
Performing loans [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		378,713	374,305
Performing loans [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		842,332	945,968
Performing loans [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		655,032	636,068
Nonperforming loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[2]	27,296	15,222
Nonperforming loans [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[2]	12,776	3,751
Nonperforming loans [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[2]	177	0
Nonperforming loans [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[2]	1,747	2,260
Nonperforming loans [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[2]	2,765	2,167
Nonperforming loans [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[2]	3,755	697
Nonperforming loans [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[2]	4,266	5,599
Nonperforming loans [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable	[2]	1,810	748
Purchased credit-impaired loans [Member] | Performing loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		38,674	58,600
Purchased credit-impaired loans [Member] | Performing loans [Member] | Commercial real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		28,544	40,985
Purchased credit-impaired loans [Member] | Performing loans [Member] | Multifamily real estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		258	1,994
Purchased credit-impaired loans [Member] | Performing loans [Member] | Construction and Land [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		4,153	5,650
Purchased credit-impaired loans [Member] | Performing loans [Member] | Commercial business [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		4,264	7,302
Purchased credit-impaired loans [Member] | Performing loans [Member] | Agricultural Business/Farmland [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		807	1,529
Purchased credit-impaired loans [Member] | Performing loans [Member] | One- to four-family [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		301	1,066
Purchased credit-impaired loans [Member] | Performing loans [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans receivable		$ 347	$ 74

[1] The Pass category includes some performing loans that are part of homogenous pools which are not individually risk-rated. This includes all consumer loans, all one- to four-family residential loans and, as of September 30, 2016 and December 31, 2015, in the commercial business category, $202.2 million and $150.0 million, respectively, of credit-scored small business loans. As loans in these pools become non-performing, they are individually risk-rated.

[2] Non-performing loans include non-accrual loans and loans past due greater than 90 days and on accrual status.